10 seconds is indeed quite a bit and I'm guessing the RS6 will be as quick as the Merc too but if you were to run all three the RS6, the E63 and the M5 for a timed 5 laps which of the three would post the best combined time, I reckon it will be the M5 cause its front grip won't tail off as quickly as the others.
But who cares I've yet to drive any I've own as hard as I would on the track.