There is no escaping it, if all things are equal except you make more power, they will make more noise at the rear. Most recent experience: Installed set of JBA's on my son's '91 GT, and it makes it just noticeably louder at the rear.
Russ
http://albums.photopoint.com/j/View?u=1595962&a=12199293&p=44358027.JPG
Russ
http://albums.photopoint.com/j/View?u=1595962&a=12199293&p=44358027.JPG