Old bows are a different story. I only use B-55 on those...I just rarely shoot one.
Again, it's not about speed. If 2-4 fps was "make or break" for me, I'd choose a different weapon. Consistency, stability, durability. Those are a big deal, to me anyway.
Not long ago I made a string for a guy I've known for a long time, but never had a chance to hunt or shoot with (we live way apart). He was die-hard Dacron--nothing else was quiet to him, especially on a particular bow he was shooting. I sent him a BCY-X string, just to try it. He's converted.