Crossfire is like SLI... if you have two monitors though, don't use crossfire. Just plug one monitor into each and you do not need any dummy plugs. The miners get better hash rates without crossfire.
Are there any numbers, regardless this?
I have 2 5970's in crossfire (4 Gpus) I have bench marked with 3 PLANAR 27 inch in crossfire with an eyefinidy dispay group resolution of 5760x1080 running various different settings ...I saw no difference at all vs just 1 monitor at 1920x1080 or even all the way down to 640x480 res 16 bit. No difference at all. getting 369-377 on all for cores at 900/500.