GTSS vs GTRS, I think it would be more of what are you chasing?
and will it be on the standard displacement 2.6L?
I reckon merli is on the right track with the 2.6L + GTSS combo, matched with some cams, lightweight flywheel, should be responsive as and be fun to drive on the circuit and street.
IMO the GTRS is more suited to a 2.7+ litre combo.
But if you're after big HP then go big single?