I never thought the primary benefit of an X or Y pipe was HP, though it does theoretically improve fuel scavenging a little. But it's more to smooth things out, reducing glug, vibration, strain on the heads, valves, gaskets, shafts, everything, and improve the general engine note especially at idle. 'Cause basically you have two times per cycle when cylinders on the same side fire consecutively, and on top of that, those occur back to back.