I have an OpenCL programs that runs about 25% faster when the screen goes into standby mode.
At the same time the compiz process takes 100% cpu usage. This is a known bug that is discussed in here : https://bugs.launchpad.net/ubuntu/+source/compiz/+bug/969860
That site suggest a workaround (enable "Force full screen redraw (buffer swap) on repaint") which fixes the 100% cpu usage issue, but the performance now stays low even when the screen goes into standby mode.
This behavior have been happened to me on ubuntu 12.04 with Radeon 6970 and 7970 with 2.7 sdk 12-4 and 12-6 drivers.
I guess that getting better performance when the screen goes into standby mode is reasonable, but right now to achieve this I must "pay" with one core of 100% usage.
I can't tell if this a general GPU drivers issue or an OpenCL one.