on a gts its about 5 whp higher which is above average. it's also slightly heavier than the stock exhaust. i dont have this for myself so i can't help you with the opinion part..but you can listen to sound clips of it (as well as several other exhausts) here...also included are dyno graphs, weights, tip diameter measurements and some other info as well
http://www.turbomagazine.com/tech/0209tur_exhaust/index.html