Hotrod Magazine's breakdown of Exhaust 'science'
Similar discussion utilizing an upgrade on a 1971 Lemans
What I gathered skimming during class:
"Functionally, the difference between the X and the H is, the X type forces the exhaust to cross between both sides. Based on the performance increases we noted with the X crossover, the scavenging forces created by the gasses crossing over in this way appear to be stronger and more effective across a wider range of engine speeds.
Whereas conventional dual exhaust systems have a characteristic rap or low rumble when the engine is under load, the X type crossover has a higher pitched smoother note."
So you're probably going to make the engine breathe better by mixing the exhaust outputs and allowing both sided of the V to use the whole system rather than one pipe per side. Also helps increase power and decrease pressure buildup in the exhaust due to all the bends made.