No real car enthusiast/modder would do that, but if they did, whats the point? Its a game, everyone can express themselves as they want, and thats the wonder with forza. On the other hand, if we talking gamewize who would want a stock car that dos not stand out from the group, or could be recognized from the other 16 players online? Telling you, gt5 has really failed on the customization scene that are expected these days.
I can agree with the customization. I would love to be able to make my cars I race online unique. But what it really boils down to for me is the physics. The game has always been about driving cars, from your normal daily driver, to the exotic race cars designed to go fast. Nothing will change this fact. Not even customization.