I have played with this some, on my black widow PSA recurve. The specs say to brace it between 8.25 and 9 inches. But it just seemed too loud to me, and then I read on another forum where a guy had the same thoughts I was having, and raised his BH up past the 9 inches. His bow shot great, and got alot quieter.
So I moved mine up to 9.25 and it was quieter, then 9.5 and WOW........still shot great, didn't move my impact that much, but oh man, it is ever a much quieter bow.
So many variables go into the setup, shooter form, release, and so on. I keep my widow braced at 9.5 now, a full half inch higher than they recommend, but hey, it is what it is, and it works great for me. I dont think I lost any measurable speed either. The arrow seems to be flying just as sweet, and just as fast.
Just my experience.