Cut out gains.
#8
#10
#13
I haven't seen a dyno of a stock car with the cutout open and then closed, but I've seen sheets on fairly modded cars with that scenario... It really depends on what else is done to the car. You'll see a much bigger difference on a heads/cam/headers car than you will on a stocker... On heads/cam cars I've seen as much as a 30hp difference. Of course the exhaust behind the cutout was pretty restrictive too. On a stock car, I'd expect 7-12 or maybe 15 if you're lucky, but that's better than any cat-back you're going to find.