The 956 did 242mph at Le Mans in '86, they had upwards of 800bhp in qualifying. The 4.5 litre 917s had 580bhp and the 4.9 litre cars had ~620bhp. A 917LH would easily outdrag a 956/962 at the top end but the 917K wasn't as quick in a straight line. Temper that the 956/962 used ground effect so they made a massive amount of downforce without much drag.
At least they have different gear ratio and not some direct drive or CVT nonsense. I'm pretty sure all you are hearing is meshing gears, but at least it's something.