Some of your phrasing looks like you think I might be a dev, but I am not, just to clarify things.
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From what I have read the XBox One X's GPU is like a 580+ some Vega features. (Though it doesn't support mixed precision GPU compute like the PS4 pro does).
It is up to the developers, and the degree of implementation will no doubt very, but I expect you are right that those trying to get the max out of the new xbox will use asynchronous shading (some on ps4 pro). They are pushing 4k as much as possible, the extra memory and memory bandwidth no doubt helping a lot in this regard, along with the more powerful GPU of course.
There are more titles in the wings with dx12 or Vulkan support. Too early to say if one API will emerge to rule them all.
Outside of consoles MS of course supports Nvidia GPUs as many users on Windows are using Nvidia.
New Xbox will help sell a number of 4k TVs for sure. Over the next year or 2 as the new console gets cheaper no doubt more will make the move. When the true next gen consoles come, probably around 2019/2020 no doubt they will both target 4k as the norm I would think. The CPUs should be a lot stronger too so they might even make 60fps the new console standard. Time will tell.