It would actually be cool if people know what they are talking about.
DX7 vs DX8 vx DX9 ...
You >can< force code to use higher DX version. Same code, different libraries.
Will it change anything? Maybe, but only for negligible part.
Main difference between DX versions is in extra features each one has.
Just changing DX version without code rewrite will have minor, if any FPS impact.
However, adding extra GFX goodies with new DX may even have negative FPS impact.
On the other hand, on server like IHR with tons of cars, I'd say bigger bottleneck could be CPU.
Somehow I doubt LFS is written with multicore support in mind.
Similar fate suffer other games, like World of Warcraft for example.
Often false conclusion is that quad core is better than dual core CPU, while it has more 'horsepower', very often in games dual-core will perform better due to higher core frequency which is due to lack of proper multicore design (code) of game itself.
In other words, I believe you'd get better FPS increase with re-coding game to properly support more cores.
However, with development as is, I do not see that as an possibility anywhere near in this century