Sorry this discussion is drifting away from the original query. Still it is a great discussion
Dome ports produce a curved virtual image. Our cameras have (currently) flat sensors. So in order to get both the center of the virtual image and the corners in focus, we have to rely on depth of field.
DX sensor cameras have inherently greater DOF than FX for a given aperture. This means that they can be used with smaller domes (which produce a curved virtual image that is closer to the sensor) and at wider apertures than their DX brethren and still produce acceptable results.
What constitutes acceptable corner sharpness is largely down to how much distortion you are prepared to accept. This is also reflected in the type of image and how you frame it. Blurry corners of large animals in the blue are probably not a problem, but wide angle reef scenics will show the issue.
The problem I anticipate is when you combine FX's inherent reduced depth of field with an increase in resolution. The details that the increased resolution produces effectively draw attention to the optical imperfections of the port/lens interface.
My observations are based on using both the D800/810, which are both 36MP FX cameras. At this level of resolution, lens choices do become restricted. I personally found the that the 17-35mm was not capable of producing corner sharpness that I was happy with (I actually find it soft at the surface too!). I did shoot the Nikon 20mm and it was OK at smaller apertures and my personal choice is a legacy prime lens, the Tokina 17mm f/3.5-4.5. In terms of fisheye, I found that using the Sigma 15mm with a 1.4 x Kenko TC degraded the image to a level that I was not happy with. I have extrapolated the effects of 10MP more and am worried that this will exaggerate the flaws in the optical performance of many lenses behind domes.
Aperture is critical too, many lenses will probably work fine at f/14 and beyond, the issue here is what happens at f/11.
FX (and indeed the D850 specifically) does give improved high ISO performance. For a given level of light, this can mean that a wider aperture can be chosen, but the increased DOF in DX systems may allow them to use smaller apertures and still get acceptable corners.
It should also be pointed out that a lens' performance behind a dome is not related to directly to focal length or optical quality. For example, the otherwise excellent Nikon 14-24mm f/2.8 performs badly behind a dome port.
My concern is that the increase resolution of the D850, combined with its inherently shallow DOF, will create problems with image quality. The 16-35mm f/4 (with a 9" dome) is the only current rectilinear lens that I found that was capable of producing corner sharpness in wide angle scenics that I was personally happy with. I predict that I will not be happy with this combination at 46MP.
Nikonos and other water contact lenses are designed to shoot wide open and still produce excellent optical results. Of course this will be at the expense of DOF. The RS 13mm is an amazing lens, but it is still a fisheye. The 8-15mm behind a 7" dome is a very capable combination that is significantly cheaper! My understanding is that there will be new water contact lenses coming to market shortly, but they are also fisheye.
The AF system in the D850/D500/D5 is so good that it is a no brainer for macro.