Windows 7 took off really well -- the only reason I see to change the engine version from A7 to A8 will be when DirectX 11 gets supported.
Jibb
DX 11 will bring you only advantages if you really use it's features, if you support real-time tesselation, global illumination, deferreed lighting or ambient occlusion. Otherwise you can do like id-software and just stick to DX9.
And then you also have to adapt the architecture of the renderer to really support shader-based rendering.
