I am having what seems like this EXACT issue with xubuntu 14.04, 2 EVGA GTX580’s and 3 AOC 1080p monitors. I have tried many different driver versions and they all fall flat on their face when trying to use base mosaic. I see from your first post that you are using XFCE with arch as I am with Xubuntu. Have your tried other desktop environments like gnome, cinnamon or kde?
Im going to shove a spare HDD in my sata dock and load up some other environments and see what I can come up with.