I assume by flash suppressor you are talking about a muzzle break? They do cut down on recoil, I do not know about velocity though. They GREATLY increase noise. If you use one for hunting, wear ear plugs or you will regret it.

I bought a Browning BAR 30-06 with the BOSS system, which is a muzzle break that adjusts in and out to tune the barrel for accuracy. The first time I shot a deer with it I felt like I had been at a rock concert. My ears rang for a while. I replaced the break with a solid piece and can now hear the action work and the ejected shell hit the ground below me. I will never use a muzzle break again for a hunting gun.