at 3600 rpm with governor intact and funcitoning correctly, you will see about a 11-12% increase in horsepower. If you completely bypass or remove the governor, you will see higher gains all else the same as the governor is limiting flow by closing the carburetor throttle butterfly.
The additional benefit is that you will be able to rev higher without going lean, which is feasible up to valve-float with the governor untouched. so instead of going 3600 rpm, you back out the screw and you go 5500 rpm, which is a 52% increase over stock rpm, which will translate into a 52% higher top speed.
The three things that make horsepower are flow flow and flow. The three things that help the most with flow are cylinder head, camshaft, and carburetor in no particular order. If you have all three its like friggin zelda getting the triforce.