Tonight I installed the newly released Catalyst 14.9.
It went in smoothly and it came up defaulted to the 10 bits per channel setting:
The bad news is that smooth gradients as displayed by Photoshop still show 256 steps in them. Ah well, maybe ATI's color depth setting means something different than I thought.
I'll keep poking around; maybe there's still hope.
The Passmark PerformanceTest benchmark doesn't show any significant speed difference from Catalyst 14.4 drivers, by the way. If I take on any new problems I'll probably just uninstall 14.9 and drop back. 14.4 was working flawlessly.
-Noel