On another forum a guy who says that he has driven multiple S8s says they are all that quick.
And he also confirms that the S6/S7 "420" hp V8 and the S8 "520" V8 have significant mechanical differences as they use different crankshafts, main bearings, pistons, compression ratios, cold side of the turbo, air intakes and others as well. Good news as the S8 and RS6 V8 should be much stouter.