Depends on the bow, most of my Schafers are B50 only. I did have one that was ok for ff but it had some handshock and was loud no matter what I did, put a B55 on it and it was perfect. My new Huntsman is ff ok, has a D97 on it right now but I do not like the feel of it, ordered a couple of Rhino strings from Allen and when they get here one will go on it, should improve the feel and quiet the bow up.
I can say that I can tell no difference in the B55 or modern string material on my Schafers as far as speed and such.