I wouldn't mess with the rear wing on a GT3 -or any other Porsche. Bad idea. If you remove the rear wing the aerodynamic balance is upset at high speed because you get a disproportionate amount of lift compared to downforce at the front. The standard set-up for Porsche is increasing understeer as speed and aero forces increase. This is a safety feature. Take off the wing and the car will become less planted at the rear and if you need to lift off the gas at high speed it may become seriously unstable.
(especially in slippery conditions. (Think early Audi TT-no spoiler Autobahn crashes
)). Unlike some other cars, Porsche fits these things for function first. Paul Frere's excellent book documenting the history of the 911 discusses the issue faced by the engineers and the solutions. I expect that the centre of pressure would also change because compared to the 997 and 997S the GT3 has more downforce on the front axle. This could aslo affect straight line stability. You may also recall on the normal 997's there is warning light if the rear spoiler does not raise at speed-that's there for a reason!