Power to weight matters in acceleration when you are talking about torque and not horsepower. If an M6 can do 0~200km/h in 13.4s with only 383ft/lbs of torque, I reckon the RS6 will be able to match of possibly better this with an estimated 550ft/lbs of torque.
I am not hyping to RS6, I am only relaying what I have been told about the car. It is meant to be an animal and it's handling is like-wise meant to be very impressive for such a large car.