I've used the Zen 170mm on the Nikon D800 and D810 with the Sigma 15mm f2.8 and Nikon 16mm f2.8.
Both the dome curvature and size are actually an improvement over that of the more circular, smaller ports and you should be able to stop down more than you can currently. You will not be able to shoot at the 8mm end of the 8-15mm as the shade is not removable.
Despite the fisheye producing a "curved" image, the sensor in the camera is still flat!
The bigger the dome and the less curve it has, the "flatter" the virtual image is. Hence is is easier to achieve sharp corners.This effects all lenses but it is often harder to see the difference in fisheye images. I am surprised about your comments on using the 8-15mm with an 8.5" dome. I wonder if the issues are to do with the TC/adaptor? I have used the 8-15mm with small ports on full frame (Canon) cameras and found the results to be good even at larger apertures than those you list.
With cropped sensor cameras, their inherent increased depth of field makes them much better at dealing with curved virtual images.
All the best
Thanks for sharing your experience, Adam! So if I understand you correctly, you're able to shoot the Sigma 15mm and Nikon 16mm with the Zen 170mm port and get good results at apertures wider than F8? How wide do you normally shoot? If the results are good at F8, I'd be more than happy -- F5.6 would be icing on the cake..
Regarding my current setup, I'm quite happy with the image quality of the Canon 8-15 + 1.4x TC in the 8.5" dome at F11. See below for an examples, without the Kenko 1.4x TC. It's possible that the corner sharpness would be acceptable at F8 or even F5.6 too -- I haven't really tried it. For my current conditions (tropical water), I can shoot at F11 and 1/250th most of the time without having to push the ISO past 640/800. I don't really notice any noise issues at those ISOs on the A7RII, and the dynamic range difference between ISO 640 and ISO 320 is less than half a stop.