Would the M5 do better with an aggressive tune and headers? Absolutely. Would the M6 do better with exhaust modifications other than being tune only? Absolutely. Keep in mind this is also the F12 M6 convertible and not the coupe which means the M6 is carrying roughly an extra 300 pounds.
It is interesting to note that in the final run where they go up to what appears ~180 miles per hour the M5 inches back. Not nearly enough to make up the ground but it shows the top end strength of the S85 V10 even without forced induction. Still, bolt on S85's are not going to beat tuned S63TU's with the power, torque, and shift speed of the dual clutch transmission working against them.
BMW M5 E60 (Mr boomerang)
RPI GT Exhaust System
RPI St Pully Kit
RPI Block off plates
RPi SV1 Oil Cooler
BMW M6 F12 PP-Performance 726hp