They don't -- I've profiled this. The only slight exception (few hundred mW) is nouveau, but it's not possible to disable the 3D without restarting X.

Even if the GPU is clocked down (ie. using nvidia powermizer?)

Well, spinning the GPU down is a different request. This does save power, but ideally we should do this in the driver anyway to save power even when on AC power.

Not necessarily... for instance- nvidia cards come with the powermizer feature that automatically clocks down the GPU when it is not in use (ie. when it is performing 2D operations)  However, when compiz is being used, it is going to put a higher load on the GPU, thus causing the GPU to be clocked higher, thus using more battery (in the area of about a watt).  More over, powermizer has a significant delay when switching clock speeds, making compiz sluggish (unrelated bug, I know, but we could certainly help out here).  Since most compiz users are using either nvidia or ati drivers, that makes a significant portion of end users who would be effected by this.  All that would be required is to put a drop-down select menu with the three options in the appearance visual effects control panel applet (none, normal, extra) in the On Battery Power tab and in the On AC power tab.

So, you're telling me there's 1000mW difference from sitting at your desktop on idle with compiz and without compiz? How did you measure this?

I'm certainly not going to make this on the profile page -- there is no use case.