Yes, you will see power increase by advancing the timing. But once you advance, you need to start running premium fuel (93 Oct). I don't know about GT's, but for the Cobras you don't need to richen the fuel.
I plan on getting a Steeda T/A and bumping up timing 3-4 degrees. A friend of mine saw 6rwhp/9rwtq simply by bumping the timing up 3 degrees.