There's the proper way to do it, then there's the band aid method. Ideally, all tubo modelling and powerband issues should be solved first, then we tweak around with tire width and thus grip to keep things on line.
Or we could simply narrow the tire of the FXO slightly and maybe add just a tad more weight, but never unitil it gets too close or exceeds the weight of the other 2 TBO cars. This would be a sort of ok band aid solution until all the relevant turbo behavior and powerband issues are solved. At least we'll have some parity in the FXO class until all the vital issues with the physics are solved, allowing LFS to more realistically depict the strengths and weaknesses of each drivetrain.
As for the RB4 dominance on the rallycross stages, that's how it's supposed to be IRL anyway, so as a sim, LFS must be no different. Besides, the other 2 cars would realistically and practically have no real business running off-road anyway.
For now, I agree with Flotch's suggestion of narrowing the FXO's tires slightly to reduce the grip slightly AND introduce some front tire wear issues RL FWD cars face. It attenuates the FXO dominance issue without relying on arbritrary addition of weight.