honestly 600rwkw and circuit, you have gotta be kidding!
think of the heat, braking, oil/water temps.
id be doubtful there is many, if any such GTR's around.
the closest i can think of was the street driven Racepace GTR which on full race fuel ran around with about 450-500, i remember seeing it had actually twisted the turbine housing.
Now think about the heat over years...
The other car that comes to mind is the Russ/High Octane which is an outright race car, still isn't 600 @ the rubber because i dare say its pointless
outright power is nothing compared to a well sorted and reliance brake/suspension/aerotrack car.
I agree the GTX35 sure does look like one the choice.