I use a 200-400 with a D500 (and an D810) it works very very well with a subject distance of less than 10m, beyond 10m the image quality decreases. Up close it is sharper than my 500mm, at distance it is not as good. I tend to use it exclusively for up close nature type photos with and without a 1.4tc, the quality hit caused by the tc is the same as any other lens.
Given the short minimum focus distance I find it a better option up close when compared to a 500mm, but the 500mm is a much better general purpose lens. The ability to zoom out makes it a much better option than any prime when used like this. Personally I am thinking for changing my 200-400 for 200mm f2, but I have been hestitent as I think I will miss it.