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The general consensus is it's subjective opinion on what type of sound you want/like. Axle vs cat may no result in any HP difference since the stock pipes are considered decent. You may be better off saving a few bucks and going with axle-back. I'm hearing x-pipe w/ cat-back replacement (i.e. Magnaflow) "may" provide some more ponies. Not sure on the tune issue for this though.