FWIW, alot depends on driver skill I think. I started S2 in the FZR. Not because it is the fastest, I didn't know back then. I never liked the GT or GTTurbo. I only had S1 for a week before I got S2, and didn't like the FXO. I played mostly with the MRT for that week at South City with the 5-6 others total still on S1 (first week of S2). Therefore, I immediately took to the FZR because I felt the others were just glorified GTT's and FXO's. Didn't even care to try them for a while.
North American Sim Racing League (NASRL) was a group that was into GTR, GTL, and rFactor. While waiting for GTR2, they decided to dabble a little in LFS. Several of us "veteran" LFS'ers joined in for a 6 race season. It was around 60/20/20 split or something new LFS'ers, some who have played LFS a little, and long time veterans. Most of the new guys took the FXR, some new and the folks who have a little experience took the FZR, most of us veterans took the FZR. The big exception was CWMAX who drove the XRR. He wiped us FZR drivers all over the track with his XRR. Dominated everywhere. Us other veterans were myself, Banshee, and TH84, all in FZR's for most of the races. Not a bunch of ultra slow drivers there.
I don't really think the GTR class is very unbalanced. It's just that very skilled drivers like the FZR. They will kick your tail no matter what they drive. It's just the way it goes. I'm always getting my rear end kicked by XRR's and FXR's. I'm also having great door to door races with XRR's and FXR's. And, I also whoop on some XRR's and FXR's. It all depends on the skills of the driver. To someone without the skill, the FZR can be a monster where the FXR floats like a butterfly. It's not the fault of the FZR, but the driver. Put me in an FXR, and I won't compete with anyone. Not because it's slow, it's because I can't drive the AWD compared to RWD. For others it's the opposite, they can't keep a RWD under them compared to a docile AWD. I'm not ultra fast, nor am I ultra slow. I just picked the FZR because in June 2005 (July? I forget) it was the sexiest car and I wanted to drive it. I then continued for a long time to get spanked by EVERYONE no matter what car they drove.