WOW! I must admit, even if the files were processed and have some noise reduction applied to them (which is what we would do anyway to these type of pictures), it is quite impressive.
Very usable! I have very few pictures taken at 6400 with my 5DmkII that look that good from a noise perspective...
1DX, 24mm f1.4L II, 50mm f1.2L, 85mm f1.2L II, 135mm f2L, 24-70mm f2.8L II, 70-200mm f2.8L IS II : D800, D4, 24 1.4G, 35 1.4G, 50 1.4G, 85 1.4G, 24-70 2.8, 70-200 2.8 VR II