As a seacam dealer you probably know more that I do about their products. I thought that I had read somewhere that Seacam had made a version of their superdome that was more of the full hemisphere. Maybe it was only the "deepwater" superdome that others on Wetpixel have referred to. My superdomes were purchased about 1999 or 2000. If all superdomes are the same portion of the full hemisphere, my good results using 80mm of extension tube with the 16-35mm lens would also be applicable for all seacam users and their superdomes.
One thing different on my superdomes is that they do not have the original seacam dome port shade of that era because it did not allow you to turn the housing on its nose to change film without the glass hitting the table or bench. So I made paira of concentric "nesting" shades for my superdomes. The inner shade is for use with a fisheye lens and does allow you to set the housing on its nose without bumping the glass. The second outer shade protrudes a bit farther and was cut to match the coverage of a 14mm lens. This shade is what I use slightly narrower angle coverage 17-35mm f2.8 and 16-35mm f4 lenses. The "larger" shade provided far better prevention of flare, particularly with the early versions of Sigma's 14mm lens which was all Nikon users had available prior to the introduction of the Nikon 14mm several years later. The outer shade can be easily removed when using the fisheye. Having them "nest" was the best way I could think of to travel with two shades. I also made pairs of nesting shades for my seacam wideports. The wider shade to use with the 17-35mm when I didn't want the size and bulk of the superdome and the outer shade cut for use with the Nikon 24-85mm.
As an afterthought - Harald Hordosch could answer this question about superdomes in a moment, and from what Stephen Frink has told me, he does watch for online comments about seacam products. But for some reason, he refuses to help in these discussions.