I only have 5 gr3 cars and have only driven them with BoP...
Merc amg gt 2016
Genesis X
Mustang
Viper
Toyota ft1
The merc feels pretty good, good balance but seems to die after 140mph..
The genesis feels pretty planted and has awesome top end, but the power/torque/boost is delivered all at once, so traction is insanely tricky..
Mustang is a decent choice, but I find the handling a little odd, the balance feels like a mid engined car, ie the front end feels too light..
The viper has good top end, but I find the front end feels too good, like it has too much aero at the front and not enough rear..
The ft1 feels the best out of the lot handling wise, awesome balance, and really lean on the car, not much torque though, so really have to rely on carrying as much speed through corners to not get jumped my big hp power/torque cars out of corners.
One more thing, The rcz needs nerfing, and asap... is one thing I've noticed. Its ridiculous