As far as I can tell, the ATI GPU2 client has not received much attention among TR folders, mainly because of lower PPD of AMD graphics cards, and partly because of the fact that the client is not as mature as the Nvidia GPU2 clients.
But the most recent cores now allow for a series of optimizations that yield slight PPD increases (10-20%), and drop CPU usage to around 0-5% on XP, Vista, and 7, like the Nvidia clients on XP. There are three or four (If you count an experimental one) optimizations that can be used. These optimizations are system environmental variables that need to be added through the Control panel.
Procedure to add Environmental Variables:
Go to start, control panel, System, Advanced System Settings, a new window called System Properties should pop up, go to the Advanced tab, and click Environmental Variables..., under the Systems variables section, click new, and then set the name (Names are case sensitive now "" needed, include the underscores), and values for the variables. Click Apply and it should be set. The GPU2 Client needs to be restarted in order for it to apply, XP may require a system restart.
The first variable is "FLUSH_INTERVAL" minimum requirements are Core version 1.22 and Catalyst 8.12 (At least 8.12 is the oldest it seems stable on). FLUSH_INTERVAL requires a value between 1 and 1024. For Core 1.22 and 1.23 and Catalyst 8.12-9.2, FL setting vary a lot depending on the graphics card used. For instance, something like a HD 4550 shouldn't need more than 64, a HD 4670 may need 128 or higher, while an HD 4870 could take 512. What FL does is reduce CPU usage, (Changes the size of packets sent to the CPU), CPU usage for these cores should gradual decrease to something around 15-25% with higher FL's. In order to set the FL start with a low value (Again lower than the values suggested something around 32 is a good start) and increase the FL in increments. With your original low FL you should have less than 100% GPU usage and only slightly lower CPU usage, increase until GPU usage is back up to 100% or so and CPU is down a fair bit.
IF you ever get a VPU recover or work unit errors, decrease the FL to the last stable value.
On the newest Core, version 1.24 and with Catalyst 9.3, FL behavior has changed, generally if you are updating from and older Catalyst start at around 32 again and work your way back up as described, in general, the new optimal FL should be less than before (Values over 200 usually cause problems).
The other two environmental variables are "CAL_NO_FLUSH" and "BROOK_YIELD". These are simple, set "CAL_NO_FLUSH" to 1 and "BROOK_YIELD" to 2. These can only run stable on Catalyst 9.3 and version 1.24!
With these two extra variables and a optimal FL, you should be able to push CPU usage down to 0-5%.
Enjoy your lower CPU usage, if your running on a dual-core computer, add the SMP client, it should run stable, or add a second CPU console client, and enjoy your extra PPD!