An immediate point is that if you have solely DirectX11 rendering, you miss out on any users still on Windows XP.
EDIT: After misreading as the project was almost finished, I think it would be good to switch to DirectX 11, since in 2 years, DX12 may be released. Though I would keep a backup option of 9 available, in case there are any xp users still.
I do not have any DirectX experience, but for the end user, there is not much difference between DirectX9 and DirectX11 features, unless they have very good eyesight and pay attention to graphics.
And a quote from the article:
What's the good news? In our testing, the DirectX 11 code path provided superior performance compared to DirectX 9 in many cases, even though the enhanced effects were enabled.