I would understand if there was an actually advantage to be gained by having custom car models, but so far I have not seen any. I have however, seen some strange things with various vob. files. One of them is that, or there was this RB4 vob file you could download, that had so much "rice" effects, so everytime there was just a little elevation change the bottom of the car would hit the ground and make the car completely impossible to controll. Even if you did make the setup as high as possible, you still would hit the ground with the bottom of the car at any bumbs in the road. Specially fun when trying to brake and you cars bottoms out and slides all over place. Other people would just see it as a me braking late, and I got a lot of flaming for it.
The other strange thing was a XRT vob I had, that had a little extra long back part of the car. Whenever anyone touched the back of my car I would be rocked launched all over track in absurd speeds, similar to barrier+lag hits, not fun with small touch ending you flying 350kmh off the track