Guys, what kind of efficiency improvement do you get when underclocking? Does someone have hard numbers?

Will power 2 rigs with a IBM bladecenter 2880W, if the difference in efficiency is low enough i'll just run 12 close to stock, or 14 down tweaked.

Any recomended brand for the 1080Ti, Blower type? I'm considering the Asus Turbo but any sugestion is welcomed.