I've had my car on a dyno with open headers and later in the same day connected to the stock exhaust from the resonator back. The difference was 7 hp. My tune has about the same advance with 19 lbs. of boost. (25 to 26º) I use 93 octane on the street and a mixture of 100 and 93 at the track.
I'm toying with the idea of changing my exhaust but for weight savings, not power. Burns stainless is one I'm looking at.

Les