when you look at fuel consumption, the fzr uses 1/3 more fuel when flat out (oval: fzr 1,2 -1,3 xrr 0,9-1.0) which is the disadvantage of the fzr on longer races. but as most of the lfs races are withouth pitstrategy at all, the fzr would win in most of the cases.
but if you have longer races with the need of pitstops, it can be handy to drive the xrr which might do pit in one time less then fzr...
i don't think a leveling is needed OR that serverside max-fuel option would be handy to level it out on shorter races.
to level out even the fxr with the others you probably need much more HP in that car. allthough the fxr has other advantages, not top speed but with handling AND damage. this sucker is massive as a tank...