With DX12 coming out later this year, where in UE4 will we be seeing a large increase in quality or performance because of it?
DirectX 12 is already confirmed for UE4, and, in fact, if you get into the “betatest” program for DX12, they give you a engine version that has DX12 support.
The increase in quality is nothing, DX12 has the same graphical features as DX11.
What DX12 does, is allow for MUCH better optimization, specially on the CPU side. Lower end CPUs will benefit from it greatly, and it also makes each drawcall be negiglible in performance cost. So if you want bigger worlds with a inmense amount of stuff, you can, as the CPU will get much less on the side.
For UE4, well, UE4 is mostly GPU bound. It will add performance for people that has a lower end CPU and laptops, and scenes will be able to have more materials and drawcalls. For ultra high end CPUs with normal graphics card, the graphics card will be the bottleneck and dont expect much more performance.
I think the main difference you missed vblanco, is that DX12 is a parallel render-er. DX11 sends all the commands to the GPU through a single pipeline. DirectX 12 makes it parallel. In theory an N-times boost is possible. (where as N = number of cores) Other than that, yeah. 
Yes, i missed that. But DX11 has limited multithreading already. Also, they were reworking the UE4 renderer, not sure if its already done, to use multithread more effectively. That was in Trello. It was something about calculating all the final “render commands” with some threads, and then submitting them with different threads, allowing for some multithreading. But adding DX12 there would definitely help tremendously.