Just read Autocar's first drive review of the RS6 is their impression of the car was glowing on everything apart from ride quality which maybe a problem in current stiffness for UK roads. Areas where the RS6 shone for them was engine, acceleration, handling, quality & engineering, smoothness and brakes. Areas which still need addressing are steering (not enough feel) and ride (too stiff for UK roads).
Regarding performance, in their opinion the car in terms of acceleration is in another league to the others, what they mean by this is anyone guess, maybe overall acceleration or in-gear acceleration but either way it IS a force to be reckoned with. Handling, they were very surprised by the handling, it's body control was exceptional for what is a heavy car and the car is exceptional nimble, changing direction extremely well and in their opinion a match for the M5, though still they feel the M5 is the more involving.
It is the first very favourable report so far, most have been so, so but as the was Chris Harris (the guy with the M3) I believe this report might be the closest to the truth.