I have the Corsa Xtreme w/ the Corsa X pipe. This X pipe fits behind the cats. According to Corsa this setup will give you 10 hp..... whatever.... I like the sound ; people have complimented me about the sound. These cats flow good enough; you'd be better off spending your coins on something else.
I have Corsa Ultimate Touring, sound sick as well. I will say that my before and after dyno just changing the xpipe was 8rwhp. Then a revised Lund tune with some hiflow cats bumped me up to a total of 13rwhp gain over just the axle backs. The sound is so sick. I really need a GoPro.