So I spent a while in the pool with the lens yesterday:
I will publish the full findings in a proper review, but the short conclusion is that it will focus on the dome port with both D810 and D500. Port was a Seacam Fisheye Port (160mm).
Another interesting fact is that even with the D810 and a 160mm dome, the corners were sharp, even at f/8
Crop of the above image:
I shot it with and without a 20mm extension. I still need to analyze the images carefully, but my initial impression is that that with this port, there was no quality improvement with it, and the vignetting was worse. Please bear in mind that this is a preliminary finding!
I want to do a little more real work testing. The lens is going to the Red Sea with Alex Mustard (well actually with Nick More, who is meeting Alex out there) soon and I may save the review until after the Wetpixel Whale Sharks trip at the end of July/early August.
For those of you that are early adopters, the quality of this lens in both build and image seems to be very good!