I had an H on a very built 6 a bunch of years ago flowing through glass packs and it made the exhaust sound like an old Jaguar. Mellow and smooth, but wide open it was great!
I was also told that on V8's that if you don't put the H in on a street car that you run the risk of valves burning. I don't know why, but that is what I remember.
Unless you have true tuned headers then the scavaging effect without the H may be reduced, hence less power.
What does the header mfg recomend?