In GT2 case answers are:

-Tires: Michelin Cups are speically designed in collaboration between Michelin and Porsche(NO specs) and they are actually also very good for normal street use as well

-New PASM: new PASM is also tuned much differently then PASM on normal 997 Turbo.

-Aerodynamics: GT2 is probably the only sportscar that is producing DOWNFORCE at front and rear. It means amazing straight line and curve stability even at very high speed. Kudos to Porsche technical stuff about this achivment since aerodynamically GT2 is a true masterpiece.

-Traction: with use of more agressive LSD that is working together with PSM it is possible to achive excellent traction for RWD only car. End result-on wet handling track GT2 is despite RWD faster more then 2s then AWD 997 Turbo...

-manual gearbox: it is pretty clear that manual gearbox is actually an advantage over DSG in above average driver hands on track like Nordschliefe. Why? Talk to some race drivers from Europe, not old farts from USA that like 997 Turbo TIP...