With ASYNC Bandwidth (faster download than upload) Can UPLOAD LIMIT prevent you from reaching your maximum speed?
I'm thinking, since every packet in TCP (not UDP) needs to be acknowledged, if you were doing a fast HTTP/FTP download with a 100MB down / 5MB up circuit (common in my area), is it possible to actually download at 100MB if you cannot acknowledge faster than 5MB?
try http://www.speedtest.net and check