you cant use the rs6-c5 discs with s8-d2 calipers because s8 disc is 30mm and rs4/6 disc is 34mm wide. the fl s8 caliper will take a disc diameter up to 380mm and width up to 32mm though.
the caliper brackets to use the rs4/6 calipers on the fl s8 with the brembo system are here
http://cgi.ebay.de/Bremssatteladapte...item1c19199ed2
and also available from urquattro in the post here
http://forum.a8parts.co.uk/showthrea...1170#post11170