So I caved and got a matching set of AP racing rear rotors 356x28mm up from the stock 330x22mm. They run a 45mm piston vs the stock 43mm piston. I also did test fit the s6 stock caliper on the RS6 brackets and they will fit and function the same and accominodate the larger width rotors, you just dont get the 45mm pistons and braking force. FYI, the RS6 brackets are not a direct fit to the knuckles, grinding was required. The C6 RS6 calipers / brackets do in fact run the stock s6 brake pad, same pad found on a lot of the larger audis with floating rears, I just sanded them down a little bit, re-used, and re-bedded. I am very pleased with the outcome and finishing off the BBKs all around. Few pics to match
RS vs S6 brackets
RS vs S6 dust shields
S6 disc weight (8k on them) vs AP disc weights
RS5 caliper plate holes drilled
Cut to fit RS6 Calipers
Test fitted
Finished Product Before and After