Looking into better exhaust for my 911 I investigated the supertrapp route.
Ended up finding a fair number of people who use them but couldn't say why
with any strength.  Others reported they sucked unless they were totally
open and thus too loud!  THat seems to be the general consensus among
non-users and past users.  I did see a test on the web were a V8 was dynoed
with several different high performance mufflers.  The supertrapp did in
fact suck, relatively!  I think it had the most noise with least power
increase.  I do think a Flowmaster was best along with  a Dynomax muffler.
It was an independent test by a magazine - look for it.

Eric