That's odd. If DX11 is more efficient than DX9 why is it that DX9 always runs better framerates? I point to games like Dragon Age 2, Crysis 2, The Secret World, and various others. Simply switching on DX11 (without turning on tessellation or any other DX11-specific features) the frames drop noticeably.
Also, does this mean the PC version is being developed semi-independently, it won't be a port? Current-gen consoles don't have and cannot support DX11 features. It stands to reason that if they shifted everything to exclusively use DX11 that they are developing the PC version with a more focused approach.