LFS needs mods as soon as possible. There are lots of mods for rFactor, and some are just great. The feeling in rFactor (IMHO) is that you have not rubber wheels but plastic ones. Anyway, some mods are very fun to drive, and some tracks are really awesome.
On the other hand I like Forza2, GT4, and probably I'll buy GT5... awesome graphics, tracks like Nordschleife, and a fair physic engine (way beyond NFS:Carbon style). Hope they include a damage model in GT5.
I have some FPS problem too, not very annoying, but it's there. I was not able to reproduce the situation where I first noticed (online race) so I had to test it offline with AI drivers.
I did try to lower the graphic settings, with/out trees, shadows, wheels, full textures, compressed... every option alone, even z-buffer from 24 to 16, but there is no difference (may be some FPS more, but not noticeable, with shadows and trees the most). The only difference was to pause/unpause the game (10FPS more in the counter).
There are some drops below 20 FPS during the race, but only in spectator mode:
The FPS may vary from one car to another, but not much (about 10 FPS more with XF GTI using the same scenery). I have noticed too that not every car has shadow, mostly if they are far away.