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the H pipe that is currently behind your pre-cats has a very small pipe that runs between both exhaust pipes. The X pipe has a much larger diameter pipe. In addition to the larger diameter pipe, the flow is improved by the design of X over H . . . just look at X & H see how the letters in X allow the flow to gradually merge the gases from one side to the other. The H is a 90 degree turn and that is inefficient in mixing both sides.
The purpose of the X pipe is to allow both exhaust pipes to scavene each cylinder . . . that increases the flow of exhaust gases . . . as for sound there will be very little noticeable difference.
you would be lucky to get 1 HP out of an X pipe,If someone tells you that you will get 5-7 HP,they are just lying,It will make you exhaust sound a whole lot better(deeper and even)less crackly-a big plus on an after market exhaust-but,I do not know If I would mess with it on factory exhaust...(JMHO)....
I had an Xpipe put on at the same time as my headers and I still have about the same amount of popping on deceleration, both with Stingers and Corsas. It's just louder now. So basically I haven't found anything that it's really done for me.