I don't have any personal experience with the BR - I've chambered several of them for others, but haven't shot one myself. On the other hand, I had two Dashers built before starting to do my own gunsmithing, and have built three more for myself, plus several others for friends since. There are also a couple of 6x47s in the safe that I've put together for myself, plus another one that I built for a friend.
My first Dasher shoots just about everything I've tried in it very well, while the 6x47s have taken more experimenting, yet still have a tough time matching the Dasher for pure accuracy at 600. That's my story, and I'm sticking with it. A Dasher will do almost anything the 6x47 will, and do it with 7-8grs. less powder. That extra powder provides a little extra velocity, but at the cost of more recoil, more carbon fouling in the throat, and probably shorter barrel life.
It may sound like I'm bashing the 6x47, but I'm only relating my unbiased personal experiences. I shot my newest 6x47 (BAT 3L, Krieger 7.5-twist, Kelbly trigger, Robertson H&H prone stock) in a 4x600 mid-range HP prone match Sunday, and was very pleased with the results. DTAC 115s at 2980fps bucked the wind well and were very accurate in HP terms. However, I'm positive I could've shot one of the Dashers with Lapua 105s and been on even terms with the 6x47 due to minimal (if any) ballistic advantage of the larger cartridge vs. the superior accuracy of the Dasher and its slightly lighter recoil. So why did I shoot the 6x47? Curiousity & because it's the newest toy in the safe. If I were going to have to make a choice between the 6x47 & Dasher (or BR) for 600yd. BR, the smaller cases would get the nod without hesitation.