I remember that first speculation set the M5 at 550bhp. Then BMW says just before it's release that it is set at 507bhp. So yeah, i could see it's power being slightly more.
@Just Me- The 507 was really the predecessor to the 7 series. In it's time it was the highest end car BMW was producing and really brought the attention that BMW was looking for for it's cars. They had made a name for themselves with their motorcycles and aircraft engines (which they were barred from producing after WW2)
The 7 series would later go on to save BMW from financial ruin and become their flagship car.
If you can find one, get it
@ Schwalbe- :bad-words: S motor?? Fragile???? HHHHHHHAAAAAAAAAAAA!!!!! I'll bet it outlasts all the AMG motors

It'll handle 700hp easy
As for Chassis Dynos being inaccurate; yes. You really have to do 3 or 4 runs and then average them out.
Each result is going to be different just given the way a piston ring shifts and seals or how much of the fuel mixture ignites (no engine is 100% effeicient and probably won't be for another 15-30 years at least). Even the clutch is going to effect the result depending on it's slip level. Granted readings are much more consistent than they were 10-20 years ago, especially since fuel injection has been introduced, but still, dynos are far from consistant.
I would love to believe that the M5 is shooting out 530hp, however it's going to take 7 or 8 different vids on different dynos to make a believer out of me.