I recently fell victim to the 6BRA phenomenon. My take, so far, is that tuning the 6BRA is many times more difficult than the good old tried and true 6BR. For 600 yard IBS the 6BR is my "go-to" and always has been. That said, when you find that 6BRA happy spot it is a thing of beauty. Just ask the guys who tore up not only the 600 but 1,000 yard record books with it in 2019. BTW, a rookie won the 1,000 yard IBS Nationals and added his name to a long list of Shooter of the Year winners this year. A feat, I believe, that was never before accomplished.
Now, to your question of velocity increase, I have yet to settle on the perfect 6BRA load but my thinking is the 6BRA falls about the middle between 6BR and 6Dasher. If not, I'm sure someone will correct me.