Out of interest do you know what exactly is suffering performance issues in DX11?
ie: Have you isolated the reduced performance to any particular system in the engine. My guess is that you may just have some features which are disabled in DX9, being enabled automatically (materials with tessellation, etc) and causing the performance cost you mention.
Generally i don’t think there is any real difference in performance between the two renderers if they are running the same feature set.
Edit: Oh i just read the report Chosker posted (thanks the for the link @O_and_N ), and he has highlighted several systems that run slower (and some workarounds).
I’m still on DX9, but fully intend to switch at some point in the future (though it’s pretty low priority for me).