Imho, the best string you can use is one of Dnk's flemish Excels w/ served on cat whiskers. here's why. The servings last forever, it's as quiet as you are gonna get, and you only lose 8-9 fps over FFF, of which his or factory shot the same fps for me, again serving life is 3-4 times factory. Stretch is also greatly minimized.
If you are shooting an Excal, you might want to join or at least lurk on the Excal forum, there's a college education on them there.