Seriously, can you feel any difference whatsoever between a non-CF car and a CF car? U2's not THAT advanced. This is a game made in 2004, and you're expecting this game to have such an advanced physics engine so it can differentiate a CF car from a non-CF car? Reality-check, liek nao.
Imagination. If anything, it makes it the car easier to spin out. That's what happened when I tried to CF my RX-7, especially when drifting. Maybe it makes it 0.1secs faster a lap in a URL, but overall it makes no different whatsoever.