I was talking with a friend who has this game, and he said the frame rate was 'perfect' after 20+ hours of play, he has never seen the framerate falter once.
Now, I am having a different experience. About once or twice (on a two lap race) the framerate while in a turn, will slip for 60 to about 30 fps. It's quick, but I notice it. This is in the single player mode, not online.
Does anyone else see this, or do I have a bad game/360?