As mentioned in the introduction, the Radeon HD 6900 "Cayman" open-source support came behind the rest of the Linux support for the Radeon HD 6000 series. There are some significant differences that left the Radeon DRM/KMS and then the Mesa/Gallium3D support landing much later than the rest of the HD 6000 family. However, it has now landed in the respective Git repositories.
It was in February that some Cayman documentation was released, in March the kernel mode-setting support landed, and just a couple weeks back the acceleration code landed. If building the very latest kernel / xf86-video-ati DDX / libdrm / Mesa, there is Radeon HD 6900 series Cayman support in place, including for this graphics card, but it's still the first-cut support. Like the other Northern Islands support, it is much slower than Catalyst and is buggy in some areas. For a high-end graphics card such as the Sapphire HD 6950, for now you are certainly best off with the proprietary Catalyst driver for the best performance, most efficient power usage, and taking full advantage of the graphics card's features. For reference, the Radeon HD 6900 series was first introduced last November.
By the time Ubuntu 11.10 and Fedora 16 roll out later in the year, there will at least be "out of the box" support on the open-source Radeon driver stack with OpenGL acceleration and compositing. Fortunately, the proprietary Catalyst Linux driver fully supports the Radeon HD 6900 series and it has done so since the Cayman launched. Testing the Sapphire Radeon HD 6950 DIRT 3 graphics card with Catalyst 11.4 releases and newer had not yielded any problems.
Benchmarking was done on an Intel Core i5 2500K setup with the Sapphire Pure Black P67 Hydra motherboard, 4GB of system memory, and 250GB Seagate ST3250310AS SATA HDD. Ubuntu 10.10 (x86_64) was installed with the Linux 2.6.35 kernel, GNOME 2.32.0 desktop, X.Org Server 1.9.0, Catalyst 11.6, GCC 4.4.5, and an EXT4 file-system.
For comparison purposes, included are the results from a Radeon HD 4870, HD 5750, HD 5770, HD 5830, HD 6570, HD 6770, HD 6870, and HD 6950 using the Phoronix Test Suite. These results are also available on OpenBenchmarking.org for those wishing to run their own graphics comparison to see how the Sapphire Radeon HD 6950 performs compared to your current system.