Here is the thing, the 599 has a detuned version of the Enzo engine in it, always has. The Ferarri FXX is base on the Enzo and had taken the engine to 750 hp and then to 800-900 hp. So the 599 XX has the same engine and puts out 700, which means it was detuned for that. And now, the 599 GTO has 670 (661 SAE) hp....all from the same Enzo based V12....
I see a Mercedes SL65 AMG Black Series vs Ferrari 599 GTO comparo.
Both have 661 hp, both are super GT cars that are lightened versions of the standard car. Except the 599 GTO is a detuned version of a rolling laboratory 599XX. Which one would win?