Here are some graphs from some local customer cars that have been tuned with my exhaust parts installed:
2002 wrx: full turboxs turboback vs Moore Performance IWG downpipe and quiet (dual 2.5" mufflers) catback, 18.5 psi, standard 16G
Gain: 2whp and 19wtq
2003 wrx: Moore Performance IWG downpipe (with cutout) and quietest catback (dual 2.5" cores) cutout comparison 19psi, VF43 turbo
Gain: 5whp and 15wtq cutout open vs closed
2005 STi: 1st graph shows a stright through catback with very loud angled "can" muffler and OTS tune. 2nd graph shows IWG downpipe and quietest catback (dual 2.5" cores) and .5 lbs more boost, with a new tune
Gain: 4whp and 25wtq
2006 wrx: Moore Performance IWG downpipe and quietest catback (dual 2.5" cores). The car made an additional 11whp over the old setup (Cobb dp and Invidia "race" catback) while drastically reducing sound levels inside and outside the cabin.
1999 2.5RS: 100% bone stock. We installed a full "quiet" catback with a chambered muffler today and even though we had an error on our baseline runs that didn't begin to show the graph until 4,500 rpm we were able to get some hp gains and some estimates on approx tq gains. We also did a test comparing a straight pipe with no muffler against the catback with the muffler and tip installed. When compared to straight pipe, the mufflered exhaust only lost 3whp and 2wtq!