As mentioned in a previous post temporal dithering on OS-X has proved troublesome with Amulet (and in fact it bedevils all KVM-over-IP systems). The guys at Amulet have written a Kernel Extension that stops the card turning on temporal dithering. James, their engineer, explained to me that it's a different technique between nVidia and Radeon, but they've got it licked;
kexstat is a utility to show which Kernel Extensions are loaded. Here I've grep'ed the output to exclude all the Apple ones. You can see the Amulet one at 0x2000
The proof of the pudding is that now all the MacPro clients on this particular Amulet system look splendid; even full-screen replay of 1080P material.