Don't know about the brakes. I am in love with the JP brakes for what I do with 6's, 7mm's and 300's.
But we just finished my first build with a Benchmark barrel. 6.5 x 284 with a 8 twist. It broke in very quickly, cleans in a few patches with BoreTech Eliminator and the first 3 shot group went .399" at 100. A little fine tuning and the rifle shot 3 into 1 hole for a C/C group of .082".
The barrel shoots so well I have almost forgotten how badly they missed their completion date and the trouble that has caused.