I'd recommend starting with a catback to see how you like it. Keep in mind that when you start adding a down and/or test pipe, the car gets noticeably louder. In fact, your best bet performance-wise is to do a catback with cold air intake. These two mods give you an excellent bang-for-the-buck and also make the car sound more aggressive. Just do yourself a favor and stay away from low-quality exhausts because they are always louder (in an annoying way) and also have a higher chance of fit and/or longevity issues.