Oh the endless discussion...
Based on all I've learned, the 5820k won't ever have a clear advantage in dx12/vulkan. There are many reasons:
- While the 5820k has 2 extra cores, it cannot be clocked as high as the 6700k and has less IPC. Single core performance is always key, and it will still be no matter what API we use. Some games might take advantage of more cores/threads, but the vast majority will keep the trend: +SC performance = better framerate.
- DX12 draw call scaling is quite lineal up to 4c/8t and it find its peak at just 6c/6t, beyond that there's no practical benefits. A 4c/8t cpu is just about 8% behind the 6c/12t version and the maximum the API allows. This difference is small enough to become irrelevant.
- The 5820k has slower cache and higher RAM latencies. They are both a key part for the draw call capacity and game engine performance now that games start to use tons of GB. Juicy EDRAM for the 19.3 million draw calls in dx12 with a Tonga and a 4c/8t cpu at just 4GHz: http://www.3dmark.com/aot/170435
Btw, the previous is a 5960x + Fury X and my modest pc recked it. Another prove more cores is not always better even in DX12.
- Consoles will keep being limited to 8 cores for quite a while. Both PS4 Pro and the future Xbox Scorpio will still run 8-core based APU. Game engines from multiplatform games will be optimized up to 8 threads.
- But the most important one, there's a point where parallelizing more the code requires a lot of work for the small improvements you could get. 8 threads are already plenty for what a game can throw (remember games code is rather linear). I don't see devs dedicating extra time and money to make 6-8 core cpus perform better than mainstream i7, when low level API are a completely different story, with all the work from the drivers now in the hands of the devs.
EDIT: Oh, and there's the price. The jump to x99 platform is not only about the cpu. Motherboards are quite more expensive also and 4-channel RAM also adds few bucks more. Jumping to the 5820k can easily become 150 bucks more.