take a capristo system with valve control and you will have a better sound and a real flap control system. or another option is HMS which is a brillant soud + xpipe....
I have installed tje capristo on mine, already a great sound. now I will install the HMS as it sounds even better (but maybe a bit much for some guys).
enjoy here:
http://www.youtube.com/watch?v=JSlfhZpXhT0
and pleae do invest in something other than a adaptation of OEM. I think that if you were able to buy the RS5 you can pay for the exhsust system right ?