No, you will not see any noticable performance improvement. A catback will likely offer mandrel bends and larger pipes, both of which are less retrictive. But the big restriction are the stock mufflers, which you already had replaced. Depending on where you live, having a stainless steel catback may be advantagous from a longevity point of view. Also, the larger pipes could be utilized if eventually adding a blower, and also re-doing the headers and mid-pipe.