Muzzle brakes do decrease accuracy a tiny bit, as do flash suppressors. But unless you are shooting in long range 700m + competitions you won't notice the difference. It's not like your nice 1 1/2" group at 100m opens up all the way to 6". More like 1 1/2" to 1 3/4" - 2".
With the addition of an AK brake on my M4 upper, it has noticably less recoil and muzzle rise than my 20" HBar with no brake/suppressor. I'd say that makes the AK M-brake well worth it considering that the M4 is about half the wieght which normally always = more recoil. It is extremely easy for me to pop off back to back shots on my M4 without losing my sight picture. I was telling a friend, it "felt" like I was shooting a Ruger 10/22.
I've never stood next to my M4 to test the side to side volume. At least to me, behind the M-brake, the volume seems a little less than normal.
I've seen some interesting up and coming concepts in M-brakes/suppressors that are actually supposed to increase accuracy. They are still in the research phase.