The 12C and 650S GT3 have been fast in GT. Transmission reliability screwed up last season but the 650S GT3 finished fairly strong in the Gulf 12 hrs and the the 12C GT3 holds the 25.4km endurance lap record at the 'ring. Basically one reason it's fast is because it uses the same restrictor but the engine is such that it can spend most of the time at the power limit at nearly all race rpms.
https://www.youtube.com/watch?v=ajN5fVVOLOw