How much HP does a greddy Catback Exhaust system add to the car?

it adds about 8 hp but the sound is the best thin in changing the exhaust

Possibly 7 or 8. Mainly it will give you a better sound. The most restrictive stuff in your exhaust is in the manifolds and catylitic converter. If you're really going for more horsepower, i suggest you look into more high performance exhaust manifold (header of some type), and a straight through cat. It's illegal to remove the cat, however there are some cats out there that are still legal, but flow really well. Check on ebay, summit etc. Goodluck.

I agree with the above....a few HP, not very much. There is no "power" inside a muffler, especially a cat-back one. The most power would come from no muffler whatsoever. So really all you're doing is getting rid of some restrictions. You have to look at a car's power train as one long tube, with an opening at one end and an opening (exit) at the other. The one end is your intake, the middle is your engine and the exit is your exhaust. If you just open up the back end and don't do anything to the front end, you really can't accomplish a great deal. But it'll sound good and you might get a little better throttle response...at least you'll "think" it has more power but on a dyno---I think 4 to 10 HP at high RPM is the best you'll do---presuming you have a large displacement engine. On a little 1.6 liter engine, maybe half that.