I've not driven the RS6, but I've driven an RS4 and an S5 on a racetrack. And that was the worst experience on a racetrack I've ever had. The RS4 cornered so bad that it made me turn my back on Audi permanently.
Same day on the same track I also drove an E63 Merc, a Lexus IS-F and a Caddy CTS-V, all of which were more pleasant to drive, corner and pretty much everything else, compared to the two Audis. The only real rival to my bimmer in terms of how I felt in the cockpit was the Merc though