The M156 V8 that the M159 V8 is based on already has a very high specific output and redline for a naturally aspirated V8 of its size. The M159 V8 in the SLS AMG Black Series is the most powerful naturally aspirated V8 in production today. The proof is below with a Dynojet graph of 510 horsepower to the rear wheels.
AMG managed to get roughly ~25 more horsepower at the wheels out of the Black Series M159 or putting it another way making it equivalent in peak horsepower to an earlier SLS AMG with an aftermarket tune. Keep in mind the Black Series revs higher by about 800 rpm meaning it has a broader powerband that can be geared more tightly for greater torque multiplication.
AMG sacrificed some torque in the Black Series for top end horsepower on the track which is what the Black Series is made for. This is likely due to the cam profile and being geared toward keeping power up over 7k rpm as the dyno shows on the Black Series M159 whereas power is dropping off by 7k rpm on the standard SLS AMG M159.
The power difference may not seem like much but squeezing power out of a naturally aspirated motor is far more difficult than in a forced induction example. Mercedes kept the displacement and the compression ratio the same. They just revved it out higher so the top end of the motor is where the main changes come in the valvetrain and cams.
Graphs below. Keep in mind the conditions for the SLS AMG Black Series dyno run are different with 85 degree warm California weather and also this is a brand new car. The 2011 SLS AMG run shows a graph from BenzBoost vendor Weistec Engineering with a before/after tune in different conditions and does not specify the correction factor but it is the same Dynojet dyno.
Dynographs couresy of Weistec Engineerg: www.weistec.com
2014 SLS AMG Black Series:
2011 SLS AMG Black Series before/after Weistec dyno tune: