I've posted the same question on a few sites. I've recieved some "information" from Jack Harrison's book and the paperwork he sent out with his bows (you actually void the warranty on his bows if you use a flemish OR any type of dacron string!), and he says he's done tests (a difference of 8-10 fps according to him), but that's all so far.
A side question--anyone ever heard of B-60?
I've seen 5-7 fps difference going from dacron to some of the newer low stretch materials, but never that kind of spread by simply going from a flemish to endless loop or visa versa. Never heard of a bow warranty being voided by using dacron. Use to be the other way around on some bows. Never heard of B-60 either, at least not yet.