Hey Frank Zappa,
I have 1999 HP500 with Eickert Exhaust, (VERY LOUD) and I put the Shotgun III's on last year and have had good luck with them. I've adjusted them to they are open about .25" static and they open all the way @ WOT. They definitely take to Edge off my exhaust note, but they won't quiet it down like silence choice or anything. I haven't noticed any power loss from them either.
Hope this helps,