This is probably old but I've been getting decent frame rates (60+) on my 6970's on EXTREME as well as no flickering or graphical corruption at all (not even in nano vision).

I'm running my 6970's on the latest driver (11.5) and the latest CAP.

Give it a try, I've had great luck with it!

My System:
1090T @ 4GHz
Crosshair V
8GB 1333
6970 Crossfire
Ultimate 64 bit

Here's the link and description:

This release of ATI Catalyst? delivers support for the latest ATI CrossFireX? profiles in a separate executable file ensuring users have access to the absolute latest set of profiles installed on their PC.

New profiles added to this release:
- Witcher 2 – Resolves missing NPI text and light source issues
- Dungeons and Dragons: Daggerdale – Resolves flickering seen when running in CrossFire mode
- Crysis 2 – Resolves image quality issues with CrossFire enabled seen the latest version of the game patch
http://sites.amd.com/us/game/downloads/Pages/radeon_win7-64.aspx