I think the double X-pipe is just wasting money. In the early days people thought it helped with drone. Most people install a regular X-pipe and are very happy. The only person I can recall installing an H-pipe is Gonzo. The X-pipe mufflers will give you less noise.
I noticed a little more power on the high end. I did remove the mufflers too so i would say a maybe 5hp increase at best.
The best way to describe the sound was...brutal.... alot louder than open headers and like a professional race car, i liked it.