Was thinking of starting 4player split screen of this on Gamecube but wanted to know how the framerate holds up.

It's been a while but I'm pretty sure it was only 2player split screen. Never really had any framerate issues with the game, always ran smoothly.

It says 4player on the back of the case, was thinkning of getting a few friends round to play it at some stage, but I can alwyas get the xbox version if its any better.

Its four player, the gamecube version runs as fine as it ever did, still quite doable.
 
I play it weekly, it holds up well.