OP, just a catback would yield nominal gains. For the twins, the biggest gain in HP is the headers, I recommend getting a catless header whether it be UEL or EL (up to personal preference on the rumblies or not). As for the OP/FP, mid-pipe, and axle back, the factory OEM is damn efficient, if you're staying NA. I also recommend reading up on this thread, it's filled with lots of info and whatnots (
http://www.ft86club.com/forums/showthread.php?t=81493)
Now for the unimportant side of my comment:
