Elite Bastards posted an ATI Catalyst 8.3 driver feature analysis

With multi-GPU configurations, avoiding CPU limitations is a very important issue at driver level - at the end of the day, enough CPU horsepower is still required to deliver frames to multiple GPUs in a timely fashion, an issue which becomes more important the more graphics cores (and thus the more frames required to be 'stacked up') you add. If the CPU can't keep up with providing those frames to the GPUs to work on, your multi-GPU scaling and performance begins to dip. ATI have worked hard to load balance everything as much as possible to avoid these CPU limitations, while also optimising the communications and calls between the graphics driver and CPU to be as efficient as possible.

ATI Catalyst 8.3 driver feature analysis