Direct from the LFS website (bolding mine):
FXO - The FXO GTR is a great car for somebody just getting used to the extra power offered by the GTR cars or for somebody who just wants to have some fun in the GTR class, but if you want wins and don't like long races you have to move to one of the rear-wheel drive GTR cars.
XRR - Is the XR GT Turbo not man enough for you? Slicks, wings, silly power and an angry face, this car has had the works done to it. Nobody knows what the LFS tuning division did to the turbocharger to get double the power out of the engine, but it works, and we are forever thankful. It likes to eat FXO GTRs for breakfast, so watch out.
FZR - This is what happens when a not-so-sane person looks at the FZ50 and says "Mmm, nice, but could do with a bit more power." 130bhp more to be exact. Like the XR GTR, it was given slicks, downforce, gorgeous looks and lightened, but just ended up better.