Hey London177,
just to let you know the discussion of MuzzleBrakes, moderators, etc are not allowed on this forum as it is based in the United States where such pieces of hardware are treated the same as Class 3 firearms...a.k.a. machine guns or full auto weapons. Please see the forum rules under the "Office" link up above if you have any other questions regarding that rule.
As for answering the question, I personally have never had a muzzle brake slow down the velocity of my airguns, but if they lowered the sound they may change the "perceived" velocity of an airgun....as in it doesn't sound as powerful, and maybe the muzzle flip is reduced a bit on a gas gun. Try running it over a chronograph if you have one available to see if you are really getting measureable differences with/without the attachment installed. I've seen guns that sound like little *_*_*_*_*_*cats shooting 10.5 grain .177 Kodiaks at around 980 fps, so I know first hand that it can be deceiving. Also are you noticing a huge change in POI? I had a muzzle brake that was clipping the pellets as they left like you describe and the shots were all over the place, I would think you would notice that more than how much penetration you are getting into a phone book. Hth!
Edit: I guess I can't say "*_*_*_*_*_*cat" without using the word *_*_*_*_*_*, which the board obviously doesn't like...oops!
