ATI has released an unsupported beta version of their upcoming Catalyst 4.9 drivers suite which will bring the first Doom III optimizations to the table. I'm hearing of some 5-8% performance boost in 9800 Pro level of cards and up to 15-20% in X800s.

In ATI own words, these drivers come unsupported and are not recommended for mission critical operations or where productivity of any kind is a concern.