power gains
they do increase the amount of air going through the motor so that should help, but most of us love the sound of our new exhaust and cant keep our foot out of it, so i ended up losing a few mpg's. also when you put in an ORY the car tends to run a little rich since it no longer has cats and the rear o2's are still there.
you can tune those out though, post up in your regional forum and see if there is anyone around you with hptuners that may be willing to help you out. thats gonna be alot more beneficial than buying one and srewing around with it until something happens
good luck bud 
