IT sounds really, really good...its not loud, deep and solid sounding.
Ive only had it installed for a week, it will sound a little louder once it breaks in...I had greddy evo on my 97 honda prelude and it sounded about the same, maybe a little louder than the celica.
there was deffinetly a horespower gain I think about 5 hp to the wheels and about 3lbft of torque (REMEMBER THATS TO THE WHEELS)
The cops dont even notice I'm there when it comes to sound, the only time the celi screams is when im in lift about 5500rpm, I dont do that around cops, you know what I mean.
This link will help you, gives you the numbers
I LOVE MY GREDDYhttp://www.turbomagazine.com/tech/0209tur_exhaust/