Re: muzzle break
1st, a muzzle brake can offer a rifle better accuacy potential simply because the rifle does not jump around as much and lessens the gas blow by as the bullet exits the muzzle. 2nd, and more important, they make YOU more accurate because you do not want to flinch and will execute a smoother shot.
That said, if you have developed a load for a rifle without a brake and introduce a weight at the end of your barrel be it a muzzle brake or a paper weight it WILL change the harmonics period. It may affect the accuracy for the better or for the worse. If the rifle WAS shooting good, then the addition should not reduce the accuracy potential of that rifle but it sure can and will change the accuracy of a GIVEN load.
It is common to have to re-tune a load after adding a brake.
Long range shooting is a process that ends with a result. Once you start to focus on the result (how bad your last shot was, how big the group is going to be, what your buck will score, what your match score is, what place you are in...) then you loose the capacity to focus on the process.