I don't see it, If you look closely at the OEM Y-pipe, it widens substantially. So the overall volume is pretty much the same. You will not gain anything more than 2 HP if anything. Deleting the secondary cats or replacing them with Hi-flow ones like Gesi might get you about 10-15 HP, Axle-back is for sound, not performance. If the muffler flow better than the stock ones, 5-6HP is your total gain.