I was wondering if you can notice any degredation in any of the games when playing 4 player splitscreen... i played monkeyball and didnt seem to notice the graphics quality being lowered like on n64, etc...
I wouldn't expect a game with simple graphics like Super Monkey Ball to experience any slowdown on a system as powerful as the Gamecube. In fact, I have not seen any slowdown at any point in the game ever, even during a 4-player race.
The case may be different for games with more detailed graphics like Wave Race. According IGN, even on 4-player mode, the game never slows down below 30 fps, although some of the finer details are lost in the 4-player modes. Of course other games may not do so well in this regard.