I have 2 6950 unlocked in CF, 2 CF connectors on 6950/6970, so you can CF 2 or 3 or 4 cards if you wish in CF, on 6990 there is 1 connector, so you can "only" have 2 cards in CF.
I did have the performance issues, when running in 16*10 resolution (1920*1200) 35-55FPS, when run in 16*9 (1920*1080) all HIGH no AA, vsync ON, 55-60, so try 16*9 res. it worked for me, and yes there must be a problem with drivers and 6900 serie.
Duckula, if you did see the review before it was pulled. In furmark the performance was lowered/raised/lowered and so on, because of the 300W limit, so the average was about 260W, so that´s why it is looking so good in Furmark