Porsche increases power out of the 3.6 liter flat-6 to 620 horses. Both cars are rated at over 600 at the crank but the GT2 RS appears to have the edge in wheel horsepower and torque clearly being underrated by Porsche. The GT2 RS is in the upper 500's at the wheels whereas the SLS AMG Black Series is in the lower 500's stock. It's easier to give conservative numbers with forced induction.
Additionally, a major difference other than the engines is the transmissions. The Porsche has a traditional manual whereas the SLS BLack Series has a dual clutch transmission that is even more aggressively tuned.
So how do they stack up on the track? Keep in mind the EVO roadcourse looks to be a bit short and tight.
SLS Black Series - 1:02.3
911 GT2 RS - 1:01.8
It's close and it will vary based on the track and driver but the GT2 RS has the quicker lap in this comparison. The lighter weight of the GT2 RS definitely helps it. At a very light 3085 pounds it is almost 700 pounds lighter than the SLS Black series. However, that is in comparison to the US curb weight which is heavier due to safety regulations. The European SLS Black Series should be close to a 400 pound difference.
Regardless, the 997 GT2 has the better power to weight ratio and despite being the older platform and not sporting a technologically advanced gearbox its raw grunt and lighter weight pull this one out.