Sorry this will be long as there is not good short answer to your question.
The real key to your question is are you happy with what you have now, if you are then don't mess with it, if not, then it may by worth and I say "may" because calibration is both Objective from a technical standpoint and in all reality is very Subjective. I have tuned a lot of TV's over the years Wide Screen, front/rear projection, CRT, and now Plasma and will say that calibration can make a big difference but in Plasma generally only if there is an issue. I will also add that calibration of a color TV, especially a Plasma is more of an art form than it is a technical skill.
So here are some things to consider and to other notes:
Get a good Calibration DVD, I got the DVE HD Calibration Disc, it is pretty good in that it tells you what all of the controls you are going to mess with do, why you are messing with them and what you are shooting for, as well as what controls you need access to to DIY, and a great array of test patterns to use while going through the process. I suggest the DVE only because it explains things very well in novice terms, comes with colored film blocks for reference, and personally of the several I have used I have found by far to be the best. That said, pick one and stick with it, so I suggest buying it and not renting one if you plan on DIY.
I have the Panasonic PZ850U and I think like your 800, we have access to pretty much the same tuning controls and mine has all of the ones you need to calibrate it, the "Pro" will not have access to anything you need you don't even if they can get to the service menu's.
There is only one reason to hire a pro and that is for their experience and to save time, I can tell you first hand that experience in tuning a picture is key to doing it well quickly, but it does not buy you much more. If you do it your self (DIY), expect it to take a few hours at least. Also it may take more then one try...
Next is when to do it, you should let your TV burn in at least 3 months before getting a Pro to tune it, that will give everything time to break in and settle down, after about 3 months the internal electronics will change in a far more subtle way over the remaining years, but if you tune it out of the box, then 3-6 months later you will need to do it again, you can almost bank on that.
Now if you DIY, then this does not matter, tune it out of the box and then hit it again in 3-6 months or anytime you are unhappy with it. Which leads me to my next point, learning how to tune the picture is a skill well worth having over time as you will find that your TV and/or room/lighting conditions will also change and all of this impacts your calibration.
Next is a good news part if you decide to DIY, you are just one menu option to always being able to everything back to exactly the way it came out of the box, the pani's all of have reset to factor options, so no matter how bad you mess it up, you can always get back to where you started. If you get a Pro to do it the first time, remember to go through every and I mean every picture menu and write down every setting, this way you can always get it back to where the pro put it if your DIY in future and you always have the factory reset option.
Now the next thing to consider is your tuning scheme I call it, the pani has 4 or at least mine does, "modes" each having their own picture settings that they remember. The scheme I used was one for Day-Time HD Blu-Ray / Night Time Blue-Ray (both optimized for 1080p), one for Day and Night 480i. The reason is that ambient light levels make a big difference the picture, and the reason for optimizing again the highest an lowest resolution was that what looks good at 1080p will not always be the best for 480i, even of you are up converting and/or up scaling to 1080p. Personally I found at the lower resolution I was better off with a little less sharpness for example and a little hotter color.
Keep in mind that on the pani's there are a couple of settings that apply to all modes, so since I really got a HD to be for Blu-Ray movies at the highest 1080p resolution, I set the few controls that apply to all to be optimized for 1080p night viewing. What you decide all depends on the source material you watch the most of and when you prefer or picture to be totally optimized, and forget about "all the time for all sources" that option went out the window with your CRT and don't feel bad, LCD, DLP front/rear, all suffer the same issue, even CRT's did but it was not to the same degree.
So now one to the last part, DVE, CNET, and any other "authority" is going to tell you that you should tune your picture to the "industry standard" and the I agree that this is a good starting point and I respectfully disagree that this is the "best" picture, just as I will argue with an audiophile that EQ of a sound system should be to a set standard. You should look at the standards for audio and video as just good starting points, unless of course you like the picture and sound when it is at the standard. The reason for a standard is that they want you to experience the video and sound just the way the producer wanted you to and my comment to that is, when the producers start giving me the video and music for free, as well as the TV and home theater system, then they can dictate the terms, until then I will put things where I like them, even if it is not exactly where they "should be".
If you get the DVE, take the hour and a half to watch the movie of them explaining it all, even though all relevant information is repeated during the calibration process, watching the movie gives you a cohesive picture, no pun intended, of what you will be doing and why.
If your pani is like mine you are lucky, of all of the TV's I have ever tuned in over 25 years of tuning TV's this set was one of the very best I ever tuned where the controls did not inter-play with the other settings too much, to some degree it is impossible to de-couple some settings from others, another reason I like the DVE disc is that they take you through the process in a very good order, and as strange as it sounds, getting the black levels and gray levels is actually the most critical adjustments to an overall great color picture. They are the reference level for everything else, so if they are wrong, chance of getting your picture optimized is very slim, DVE take you though getting that right first.
Now the not so lucky part, not matter if you have a pani or any other brand, the picture controls to some degree are interactive, that is, you mess with the contrast and that forces you to go back an mess with say the color temp or tint. This just can't be avoided it is the nature of the beast, so proper tuning of a picture is an iterative process, it is not 1,2,3, it is more like 1,2,1,2,3,4, 2, 1, 5, 6, 7, 2, 1, 8, will you get the picture, as you mess with things you will need to go back and see things done before are still right. This is where the experience helps a lot.
Now that you have a picture that meets industry standards at the current light level you did the calibration in, there is more, sigh, you are very unlikely to get these other conditions address by a pro unless he/she is a family member will you hang out all days and in to the night, as well as tune for each of your input sources.
This brings me to another sot of bad news point, every source, you game set, your Cable/Sat/Antenna, your Blu-Ray, your old DVD player, your VCR, your Tivo/PVR will all impact the picture a little different and I know of no TV on the market that will allow that many pre-sets and tune to every input source for every lighting condition, although there are some that at least do it for every input source, the pani is not one of them. However don't despair, just get used to the idea that you can't have perfect for all conditions and after a short while of watching a given TV from multiple sources you'll just get used to it and won't care and likely not even notice.
Now comes the "art form part", ok you have it to industry standards to the best of your ability or the pro got it there to the best of theirs, but you look at the picture and think, yuk, or perhaps more subtle, is that the best there is? Well the answer is no, now the reason they put all of these controls on your TV is so you can set you TV to standards, but they also allow you to set to personal preference. Very much the same as the tone/EQ controls on your audio system, there is 0dB and there is what you like
OK, not make sure you record every setting you made, this will now be your new baseline, then go through and tune it to what really looks good to you, you may find you like a little hotter or cooler picture, you may find you like it more or less sharp, same for contrast, color balance and so on, go head, tweak it to your liking, it is you that parted with the 1-5 grand for the TV, might as well get what you like out of it.
Once you are happy with the calibration in the current lighting condition, say you tuned it first in the day (I suggest that since doing this tired is never a good idea) Next you will want to repeat all of the cal steps in a darker setting the light level you typically watch ha move or TV at in the evening, on the good news side your pani has 4 sets of independent settings is will remember. Once you are happy with that picture comes the last part.
Get out a couple of movies you really like and have seen a few times, put them in and see if you like your changes, you might find that you want things a little different and while you can't effectively calibrate using a movie or TV show, you can tweak it a little while watching one.
Now one last step, look at your picture against all of your input sources there is where art, technical, and real life meet compromise. Personally, as I said I optimized for peak on a good, new, Blu-Ray movie, in native 1080p mode, in the evening and day time, us 2 of my modes. the other 2 I did for TV from my Set-Top box which for me was 1080i since I have cable, one for day and one for night. All other inputs may be less than perfect, but after a short time, just a week or so for me, the picture looks ideal from all sources in all normal viewing lighting conditions.
In the end what you want is a great picture under as many conditions as possible and it makes sense that you pick the conditions where you use your TV the most, you may watch mostly Standard Def over-the air, if that is the case, optimize it for that.
Just remember, that after a short time it will not matter much, you will get used to how things look and except for the few perfectionists in the world most of us just don't notice subtle changes and that is all they will be, subtle. What will not be subtle is top of the line newly mastered for Blue-Ray 1080p movies vs, broadcast 480i/p even if you up convert or upscale it to 1080p, if the data is not there it can't be added and the data is just not there in the older stuff to give you more, this too you will get used to.
So bottom line, I would not hire a pro, chances are that you will not feel you got enough to make it worth the cost, and if you DIY, yes it is a pin the first time, but each time you do you will get better at it and it will be easier, and then you can do say and annual tune up, that is the most often I would suggest, personally I usually don;t re-tune (except after the first 3 months) until I start noticing a difference to the point I know it can be a lot better, and I have not had my pani long enough for that to happen yet. I have tuned mine twice now, once out of the box and once about 4 months later, that was worth.
Sorry for being so long winded, but I think this is a great question people ask and rates a detailed answer so you can make a really informed decision.