I have one rifle with a brake, that I sometimes use. It's a 375 H&H AI. The brake helps when I'm shooting quite a bit.
I bought a 700 280 mountain rifle, a brake was cut into the barrel. It was the worst shooting Remington that I've ever had. I replaced the barrel with a take off and now it sub MOA all the time.
A 6.5 300 probably won't kick that bad, but a brake should work wonders as a good bit of the recoil would be the expelled gases.
I would try it both ways. If the POI doesn't change much, working up loads with the brake on would make sense, I suppose. I wouldn't hunt with a brake unless I had electronic ear protection. I shot that 375 H&H AI once without protection and my ears rang for a while. I'd bet I lost some hearing that day.
