Personally I don't think AWD can beat RWD driving dynamics.
If you like doing straight line acceleration at a drag strip then RS6 is your car. If you like to drive a technical race track like PBIR or Road Atlanta, then the M5 will beat the shit out of the RS6.
2014 ///M5 Individual Frozen Grey, Silverstone Full Merino, Piano Black, Exec Pkg, Drivers assistance plus, LED Pkg, Bang and Olufsen, 20" 343M Forged Alloy Wheels
Countermeasures: Bel StiR+, Laser Interceptor Quad HP, Zaon MRX
Upgrades: Eisenmann Sport with M Performance Carbon tips, Dinan anti-roll bars