I think there's a little misunderstanding about the function of the "H" pipe, and therefore the usefulness of the "X" pipe and it's relationship to the "H" pipe.
First of all, I hear too many people talking about "H" pipes as if they were designed to actually FLOW any gases. They aren't. They are only designed to equalize pressures, that's why they are only designed to have a small hole in them as opposed to being the full size of the actual pipe used in their construction. For this reason, their shape and right angles mean virtually nothing. To compare "H" pipes and "X" pipes is actually not a fair comparison. They are designed for two different functions. Well maybe not totally different. The "X" pipe is designed to cross-flow AND equalize, whereas the "H" pipe is only designed to equalize.
The "H" pipe is mainly installed to smooth exhaust pulses and this mainly affects the exhaust note. The "X" pipe is mainly designed to aid in total flow but really only barely out-performs a basic "H" pipe. It's strength is in the tuning of the exhaust harmonics and tone.
Bottom line, there is virtually no performance based reason to install either "H" pipes, or "X" pipes but if you want to smooth out the exhaust note AND pick up a TINY amount of power, the "X" pipe seems to be the way to do it.