There are axle back kits out there, I believe Tanabe and racingbeat are both axle back solutions. However if you are planing on doing further engine modifications then the increased size of the piping will be a good thing, however if you are going to stop at just the exhaust then no, you don't really need the full cat-back system as by itself the system won't get you any real power. That said, IMO the Magnaflow system sounds the best out of the ready bought options.

 

You can just buy the muffler and it'll sound good as well. I still have the stock CAT on my 6 as well and it sounds great. I am looking into getting the magnaflow cat on it.