You can't quantify the improvement easily, not unless you have an instrumented plane that can record accurate speeds and acceleration. So, most folks just try a different prop. Like Silversurfer said.....when you do switch, you'll wonder what happened....

I've been using only 2 props on my Brison 3.2, and I like 'em a lot.
Menz-S 22x8 and Mejzlik 22x10
On a 19 lb Double Vision biplane.....unlimited vertical