Interestingly enough, I have one .300 Win Mag in a Remington Sendero SF, and another in Winchester Classic Stainless with BOSS - so I can give you a REAL opinion.
Both of my .300s are very accurate. In fact, with PMC 180 grain Barnes X bullets, the Winchester with the BOSS shoots a bit tighter groups than the Sendero. Then again, this could also be because one gun likes a particular bullet and load better - regardless of the BOSS system.
As for recoil, the lighter Winchester with the BOSS kicks about the same, if not a little less, than the heavier Sendero without the brake.
Zachary