The reason an x pipe is an improment to exhaust flow is that because the firing orders of v8 engines are not symetrical side to side has a gap in the positive pulses on one side at same time as a lull in the oppsite side---the old racing flat 180* cranks solved this problem but created others---

as the exhaust pulse gets beyond the tunable range of the headers/collectors its just looking for a way out to the atmoshere----------so from the X to the end of the system---beyond mufflers/tail pipes------the exhaust can go down both pipes at a much steadier flow/pressure instead of being pulsed -lay out your firing order on a piece of graph paper and it will become clearer than what I have written-------
for example chev firing order

1 3 57
84 6 2

you can see the lull between 1 and 3 on the left as rt side has double of 8 and 4, 3 and 6 even side to side while then left side gets double of 5 and 7------I hope this heps make it a little clearer about the differances-----