I only use one monitor at a time. I've found a workaround, I've made three diferrent xorg.conf files, each one for one monitor and a script which prompts for "select monitor" on which the startx/kdm will start
It's very basic but it works. And also I've turned on "Unredirect fullscreen windows" in General options tab of Compiz, that solved occasion laggs in full screen videos and opengl screensavers.
Btw. when I switch video driver in (s)mplayer to vdpau the cpu is on 3-4% and I see the vsync is not working. If the video driver is set to xv or gl(2) the cpu is at 22% and I see NO vsync problems. It appears that vdpau and compiz has to be polished some more in that area and default deiterancing filters of smplayer doesn't work with vdpau, which is ok because they are related to xv/gl but vdpau deiterancing filters perform worse than xv with double framrate yadif for example... So for me it's xv,with cpu at 22% with which I can live, and also vsync works and everything is really smooth and very quick performing(Gentoo is the fastest of them all