@Aradar - You appear to be having my similar results. When you say add 2 clients windowed, do you mean on the same screen or one copy of WoW on each screen? Do you currently with your 5 copies on one screen with Maximizer or do you just alt-tab or whatever between the windows?
2 clients on one screen or multiple screens, doesn't matter. As for my setup, I use the maximizer built in to Keyclone.

My slow frame rate appears to be related to the second screen. I tried a single screen solution where I put my main on the top of the screen at a resolution of 1280x560 stretched out to 2560x1120, then my 4 slaves across the bottom of the screen at 640x480 each. This works perfect. I get a smooth 60fps on my main. I'm still a little puzzled why I can't run the 4 slaves on the 2nd monitor without affecting the main's FPS though, especially since I'm running them off a separate card.
I use this same setup, one on top, 4 across the bottom. For the most part, I get great fps on my main but I have the main’s video settings at default and the clones are set at minimal settings with an fps max of 15. I just recently got in to Scholo and Strath though and they both lag pretty bad for some reason so I have set my main to minimum settings as well.