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 Porsche 911 Turbo S
2015 Mercedes-Benz C300 Sport 4MATIC