I have the OBX muffler, for 110 on ebay a year ago.

I took it b/c i like the dual square tip and i didnt have any money for catback.

anyway, any muffler shouldnt give much power gain.
It's been ok now, except some of the layers inside the muffler may fall off and it sounds a little louder. but thats it...
i'm saving up for custom piping + high flow cat now