From this it would appear that the D5 indeed has less dynamic range in the low ISO(s) but it's optimized for high ISO.
Right, the fast cameras typically are not the best in low ISO dynamic range. As far as I know, this is partly because they need to read the sensor very quickly to achieve their frame rates, and partly because the pixel count is smaller so the noise that comes from the reading of the sensor and digitization of the signal contributes to a larger fraction of the whole image than in slower, high resolution cameras. The difference between D4(s) and D5 doesn't seem to be so great, one is a bit better in one area and the other in another. I guess what Nikon did was to make the D5 a more specialized tool than its predecessor was; on the other hand the pixel count has increased (slightly) so that increases usability for some applications. I would just use the D810 for low to medium ISO work and the D5 for medium to high ISO and/or fast action. The D5 has the newer AF system advantage, excellent build quality and ergonomics, high speed etc. whereas the D810 has the better image quality (except at very high ISO) and is more compact and lightweight (without vertical grip).
As for the choice between D4s and D5, personally I think in a camera that is intended for action, the better AF system is more important than small differences in image quality. However, there is also the matter of cost - the D4s and D4 should be available on the second hand market for less money than the D5. The biggest difference in the D5 AF system for me is that it has cross type sensors further away from the center along the long axis of the frame, this is a big deal for vertical photographs of approaching people (vertical portraits as well, but if they are approaching the new AF system really helps catch onto low contrast features on the faces). Unfortunately the AF array does not seem to have expanded as much along the short axis of the frame.
I guess if you shoot action in very bright, high contrast light a lot, then the D4 or D4s ought to be considered alongside the D5. But for contrasty daylight I would prefer the D810 and just accept the slower frame rate - 5fps with a fairly spacious buffer works for me, and it can do 6fps in 1.2x crop mode. D810 at ISO 64 is just delicious.
Any comments on the viewfinder? How is manual focus on the matte focus screen. I have not found the D800 and D300s to be as good (as easy) as my D2H was.
You could try the Df and D810; both have rather nice viewfinders (I found the improvement from D800 to D810 noticeable in the crispiness of the viewfinder image). I haven't used the D5 enough to comment on that but it may be it is easier to see the corners of the viewfinder with glasses on than with the D810. I am not sure about manual focusability with the D5 but will at some point try. I use quite a few manual focus lenses for tripod based work but use live view to finalize the focusing. For tilt/shift lenses I focus first optically and adjust the tilt simultaneously while rotating the focusing ring. Then I finalize the focus using the LV image zoomed in. You can get some viewfinder ocular accessories for the D5 (it has a removeable eyepiece and the manual lists some options that can be attached). I haven't tried those yet.
I can try to provide feedback on the D5 viewfinder for manual focusing vs. the D810 once I have more time with the camera.
