I don't like carboning a car either,but in UG2 you have to have a certain amount of style points to get magazines. Carbon fiber raises your style points. UG2 pretty much requires you to build a freaky ride.
Odd. I still don't feel anything. I'm actually trying to, as well. There is one difference, though. The steering suddenly feels...numb. I can plant it where I want, but the responsiveness is exactly the same. Acceleration is only a teensy bit better, but it lags out of the corners. Never got that before.
After that, I suppose it would be incorrect to say it doesn't make any difference. What I can say safely, however, is that the Carbon is more for the show, than it is for the door.