| I doubt you'll notice a difference. Why not go to stock and then put a GTT reroute 'one ball' pipe on to reduce restrictions and weight on the stock system. I think that GTT do this section for about £60-70 and I know that places such as ThinkMINI can fit them.... or in fact a decent local custom exhaust place could do it. Will give you a little more burble than stock without loosing you power.
TBH I know someone who had a blueflams and he didn't rate it for the power on the S | 
