More polygons won't make anything look better realy, and all it will do is increase rendering time, which will decrease frame rate.
LFS's cars are already pretty high poly as far as I'm concerned. The hoods of the cars even have edges instead of just being textured to look like they are seperated, they actualy are.
The one nice thing about LFS is that the cars are made well enough that the lighting can be done completely in the game, so you don't realy need the shading in the textures. This is something alot of other games don't have, they ususaly use alot of textures to make out the details, where LFS uses actual geometry to add the same details.