If your definition of power gains is, substantially increasing horsepower & torque, rate of acceleration, and the important fun factor, then yes you will gain lots.
Generally you’ll see a 5%-20% increase over stock depending on your application, but like jetting there are many variables to contend with. Don doesn’t put in countless hours of R&D time and lost sleep into a product to not have it perform like the way he feels it should.
We’ve been around long enough, that you can trust that any time you install an FMF exhaust product, you will definitely ‘ FEEL THE POWER ‘.