MSH, we tested a lot of glass packs and the non louvered ones flow a lot of air but as soon as you get into the cheaper ones, yep, the rate is less.
You can't go by only the air flow rate for sure but there definitely is a good correlation between excessive back pressure and reduced performance of the engine at higher speeds. Exhaust is not a smooth flow but we see a good trend between high back pressure (low flow rates) and less than perfect performance.
On powerful engines, it makes a big difference; on less powerful ones, not as much.
With the X instead of an H, we saw reduced back pressure at the engine which will let you run more restrictive mufflers (quiet or loud, take your pick) with less performance degradation. I use it for sound control, others might use the X to let them run loud nasty sounding ones. :):) The X gets rid of more resonation in most cases too, depending upon the muffler, engine's power, and length of pipes and how the mufflers and pipes mount.
Does this make sense?
Tom