I picked up 20 rwhp with the Kooks LT`s and the off road x pipe.
Brent at Brenspeed told me that I should gain at least 15RWHP but 20 was very possible depending on the car. They will also free up a little more on a supercharged car. No matter what, they are a plus. I have not had my car on a dyno yet. I hope that I,m doing 300 at the tires and pushing the 12 second zone. I do know that my car is way quicker than stock. I might try to get it on a dyno before long.