I think they say it's worth 23 hp on 98 and under cars with the 6 cats. That's for an O/R Prochamber. I always have trouble believing numbers like that. I put mine in 3 weeks ago and gained about a .1 at the track hitting a new best of 13.742@102.537. You'll enjoy the sound in the higher rev range .
With your car listed as mostly stock. You'll noticed the sound, a slight drop in low RPM torque, and a slight improvement in upper RPM Power. If you haven't gotten gears yet get them makes the porchamber so much more enjoyable. With my stock cat(less) back at idle you can tell something is differnet but not sure what. At WOT in higher RPM's it is music. Last night me and a buddy had gone out to eat I was headed home he wsa headed out an Ecplise start some trouble with me so I layed into it my buddy had to call me and tell me how sweet my car sounded taking off (he drives a fox with HCI Offroad H and flows) He said he could hear when she hit about 4500-5k range the sound changed and it was just loud bassy noise.
Its noticable, last 2 weeks ive been driving without it in. The car reved up alot easier with it in and pulled harder, with the stock pipe in it does seem to have better low end though, but it could be my imagination.