Having seen a RS4 in action at Texas Motor Speedway I can say it's a beast! On the the twisty track (inside the TMS oval) you could hear the turbos spin up from a hundred feet away, everyone stopped and watched the RS4 when they heard the turbos.
The owner of the RS4 purchased the car in Mexico City and drove it up to Texas for the Audi Quattro Challange. I have pics of the car, I got to sit in it and I had dinner with the owner. The RS4 Recaro seats are awesome.
I'd bet my money on the RS4 vs a C5 (no mods) Z06 vs RS4 would be a good match and the RS6 vs Z06 would be very close.
I wish the RS4 didn't cost so much or I'd probably have one, the wife feel in love with it when she saw it.
I still like the Vette's, I always have and someday I might own one.