I think its supposed to add somewhere around 16 Hp. I've never had mine on a dyno so I can't say for sure. The weight reduction is huge...around 20 pounds less than stock.
speedwerx website says 16 hp from the pipe and 8 hp from the y-pipe. I'm assuming this would be with reeds and air box and fuel controller. I just set mine up and it sure feels like an extra 24 hp. Slp also claims 24 hp from thier complete exhaust set up.