As documented at https://docs.unrealengine.com/udk/Three/DirectX11Rendering.html, you can pass `-DX11` to use Direct3D 11 as of "QA build March 2011", instead of the default Direct3D 9 option. On my computer with a game I tested (Viscera Cleanup Detail, which uses UDK 2013-07), this significantly increased my performance and was required for the game to remain smoothly playable after uncapping the framerate as detailed in the "Smoothed frame rate" section. I observed no glitches, bugs, crashes, or any other effects beyond increased performance.
I did see an anecdotal report from some years ago for a different game (I don't remember what) that switching from D3D9 to D3D11 decreased performance, so it should be noted that this is not guaranteed to be effective. I also figure it may break some games that were written in ways that assume D3D9, although I haven't tested this or seen any reports of it.
