I have never seen dyno sheets before, one thing would have been good if they also ran the baseline of neither... I have a Dr. Gas X-Pipe on my Mustang and could not tell you if it makes any more power vs. H-Pipe or nothing, but I can tell you that it does sound very good (GT40'ish with the bundle of snakes). I also had on an old Big Block Chevelle a H-Pipe and it seemed to make the car a bit quieter, but nothing empirical. Would be good to see other dyno charts for sure.

I don't have any experience with H-pipes but I do have 2 old cars with a X-pipe (one has 2.5" exhaust, ther other is 3") .

The car with 2.5" had the X-pipe added recently and the change in sound was very dramatic in my case. The car is much quieter at all speeds. At WOT the car is almost too quiet for my taste. previously this car had an annoying resonance at a steady cruise speed of 2000 rpm and that was also greatly reduced after the addition of the X-pipe. I bought my X-pipe from www.ramairrestorations.com. They claim that all X-pipes are not equal.

Some folks claim big power increases but my seat of the pants feel is unchanged. Due to the reduction in noise it almost gives the impression of being slower than before but the timeslips from the track will tell me the true story this Friday night.

The biggest drawback of the X-pipe may be the issue of clearance under the car depending on the location of the transmission crossmember, driveshaft loop, etc.