I've not done this and clearly the guy who has is involved in the car trade, but he's converted his S6 front bumper, wings, wheels and rear diffuser to an RS6. it does look good...
http://www.audiworld.com/forums/s6-c7-platform-discussion-199/c7-rs6-front-bumper-2850197/page3/
http://www.audiworld.com/forums/s6-c7-platform-discussion-199/c7-rs6-front-bumper-2850197/page3/