The RS7 is around 550/560 BHP, unsure of the weight but Prob more than a C63. My C63 is running eurocharged map so Prob around 500 ish bhp + secondry cat delete and Quaife ( not that it's relevant )...
I know for a fact the RS7 driver was going balls to the Wall as he overtook me at 100+ and was weaving in and out of traffic like an idiot, this is before I even started going for it myself. Also the "race" was a rolling start at high motorway speeds... I'm not saying the 63 would have overtaken, but the RS7 couldn't pull away cleanly and to be fair I was stringing out every last Rev.
End of the day the RS7 will be faster in probably every circumstance but when rolling at 60 mph + there's not that much in it, fact!