It wont alter the physics of the car, but it would alter things like collision detection. If you were to remove say, the front end off the XFG, then a collision would no be detected until the shorter front end touches that contact.
If your online however, and the guy your racing has the original XFG .VOB, then on his computer, it would look as if your touching his car and you would be knocking his car around, yet on your side, it looks as if your not touching.
Thats why its very dangerous to use altered .VOB files online, especially when racing and bump drafting, as even the small MM in change can make a difference.