Well, I did noticed something like a 20-30fps loss when switching to dx11 as you said.
I did noticed this:
https://forums.unrealengine.com/legacy-tools-unreal-engine-3-udk/udk-content-creation-and-design/96628-weird-shadow-in-dx11-amd-a-quadro-k4200
What looks like is that the ut3 weapons are rendered in front of the lightning.(not sure how your system is build).
In regard to displacement.There was a long exhausting topic here with me and
https://forums.unrealengine.com/legacy-tools-unreal-engine-3-udk/udk-content-creation-and-design/85574-help-with-tessellation
Looks like no one has managed to make a shader that limits to a number the displacement amount, which goes to infinity based on mesh polycount.Why I say based on mesh polycount?
Well, I did noticed that if I place a displacement map(lets say a brick shader) on a simple 4 sided polyplane,that one will subdivide to the necessary amount and stop as it shoud.The problem comes when you give displacement on a higher polygone mesh.The displacement just keeps on adding geometry.
So for now I’m using POM shaders.If by some chance you discover the grail of a displacement shader that has a actual limit of tessellation, please share it with your udk brothers 