Like most on here have stated; it varies from bow to bow. I purchased an eight strand D97 FF string from a popular string maker. I tried it on my 60" widow recurve, and couldn't get it as quiet as my dacron strings. The dacron is noticably quieter.
I also hear about the increase of speed. That is very likely. I never checked it out. I do find it funny that people will front end load their arrows to get an EFOC, because they are not concerned about speed, but usually want the 8 FPS increase the skinny string will give them.