Burgy-
A "test pipe" or "dump pipe" just replaces the catalytic converter with a solid straight-through design pipe. People say (and I'm one) that if you don't want to run the cat, and say punch it out...(cheap fix for small gain in flow) you create turbulence in the cat chamber because it's got that irregular shape -(wide in the middle, narrow at both ends) and that will decrease your flow anyways, and SOME people (not necessarily myself) think that the increased turbulence is actually BAD for your system.
I run a PDM test pipe, and I'm very happy with it- I don't get as many backfires and such, although I really wouldn't say it's any louder...very simple install also.
YES, you now will have exhaust emissions problems (NO CAT!<img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/wink.gif" border="0" valign="absmiddle" alt=';)'>
I noticed a decent power gain, but I could also be smoking crack. I would say if you're not interested in saving the world from catastrophe, try one on for size, just make sure you have friends in the inspection industry. You should also be able to run the test pipe with a turbo, I don't see any fitment difference...
Welp- hope that helps a bit... <img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/dozingoff.gif" border="0" valign="absmiddle" alt=':zzz:'>