As stated above. Axle backs aren't going to give much, if any performance gains. Axle back exhaust bolts up immediately behind the rear axle. Cat back exhaust will provide more performance gains than axle backs, but still nothing extreme. If you want to improve performance significantly through exhaust mods you'll need to replace the entire exhaust starting at the headers (where the exhaust leaves the engine block). Look into long/short tube headers, a new mid pipe either with or without cats (without for the most performance, but at a cost if your state requires emissions testing), then cat back systems. Use the search function for forums that discuss the various options at each stage of the exhaust to find what you like best.
2011 V6 Kona Blue, MT, 3.55's, airaid cai, 93R tune, BBK shorty headers, 2.5" exhaust, GT axle-backs, BMR rear LCAs, pony package, comfort package