I would say no, you really woulnd't notice a big difference at all in gameplay (infact in theory you shouldn't see ANY). Connecting 'may' be a little faster, but not gameplay.
I've got 8Mb down and it's hardly reflected in online play with the Wii. Considering there are so many different line speeds around the world nintendo obviously had the write the protocol so that people with very slow internet could still play without lag etc (seeings as the games don't show a latency measure), otherwise this would lead to an unfair advantage to the person with faster internet (as it does in PC based games).
So in my own opinion, your online play should be almost identical whether you have a 256K line, or an 8mb line. It may just connect a bit faster, tho you still have to wait for others to join a game which has nothing to do with your own line speed.