Honestly, if you really need the AWD, I would consider a B8 S4 before a B7 RS4. Like others have said, the DRC and carbon buildup were major issues in the B7 RS4 and the performance hit going to a B8 S4 is not as drastic as you would think. The B8 S4 is a beast for what it is and what it competes against (335). Software upgrades to the S4 would get you near M3/RS4 power but Audi is cracking down on mods so you might have issues with warranty if you go that route.
09 E90 M3 - Interlagos Blue / Black Novillo / Sycamore Anthracite / Eibach / HRE