X-pipe reduces the rasp. Straight pipe is always the best performance wise, Shiv is right on that. You want to get that Air our of there as fast as possible/shortest distance of travel, increase your performance. Just think about how breathing works for your own body.
(Breath in more (intake) release more as quickly and in the shortest route(exhaust), now with your body its through your mouth rather then your nose, you do this when you are running for maximum performance- your body needs more oxygen.) Heh sorry guys thought it be fun to use the body as an analogy.