Problem solved without any modifications in PS3 settings
I confronted the same problem with my PS3. It was not from the start. Just appeared one day. None of the solutions found on forums worked for me and I refused to drop the wi-fi option and to use a wired conection.
Finally I managed to have back my 17 Mbps download and 9 Mbps upload connection on my PS3. And, ofcourse, my MAG sessions
The problem is not in the PS3, but outside. Look in the proximity of the PS3 at other electronic equipment that might interfere with the PS3 wi-fi signal. In my case it was the wi-fi TV&Video transmitter. I moved-it 35 inches to the left and the PS3 wi-fi connection worked again as a charm.
I intend to post a demo video to prove how much this thing is influencing the PS3 wi-fi functionality here: