DX9 is faster than DX10, which is faster than DX11... (Running the same commands.) DX10 just adds more commands to DX9, and nothing special. (It is also only available on Vista and Win7, not xbox or XP.) DX11 adds a lot of coolness, at the cost of near-death of the games... "Crysis 2" Average FPS is about 20FPS across the board, unless you turn off the effects, in which case, it is reduced to DX9, and still runs slower on DX11, than it does on an XP rig, with the same DX9-only settings. DX11 is a Win7-only option. (You now exclude X-Box, XP, and Vista.) However... If they actually used "tessellation", and used it correctly... (Thus removing all the LOD junk from models.) The game would be a LOT faster, and easier to manage... If "Tessellation" actually worked like it should, which it does not, still... Other than that, DX11 offers nothing special. Great looking water, skies, and vines... But simple programming can/does surpass those "auto-features". Shaders... Now there is something that they could use. Since they keep trying to code shaders, and do it horribly... Using DX11 would just be even slower, unless they actually use the DX11 shaders, correctly. Since they can't manage DX9 shaders, I fail to believe that they could manage the more complex DX11 shaders code. Obviously, someone needs to take a shaders-class.
DirectX 9 is not faster than DirectX 10, which is not faster than DirectX 11. The newer version allow more to be done on the GPU versus the CPU. In the past few years GPU power has gone up at a faster rate than CPU power. If the physics calculations, shadow calculation, and geometry acceleration can be offloaded to the GPU, it will allow faster rendering of the same effects, or higher quality rendering of the effects at the same speed. Direct X 10 for example allows Instancing 2.0 which means "allowing multiple instances of similar meshes, such as armies, or grass or trees, to be rendered in a single draw call, reducing the processing time needed for multiple similar objects to that of a single one" per wikipedia. Imagine the number times the same tree and strand of grass are drawn in Skyrim multiple times on the screen. Currently under DirectX 9, each instance of this has to be sent to the GPU over the rendering pipeline. With DirectX 10 support, the game could send the unique mesh for each type of tree and grass once, and then tell the GPU where to show it many times in the scene. This would speed things up with the new version, not slow them down.