I spent most of the last three months shooting with a Nikon 8-15mm Fisheye lens courtesy of Ikelite, who so kindly let me borrow it to see how it shoots behind both of their dome ports (the 8inch and the compact 8inch dome). It was excellent (and added to my Christmas wish-list) and it did well with both ports. I took it to Bikini Atoll, Roatan, and Tiger Beach.
We knew it would be great behind the large 8inch but we wondered if the compact port would cause the edges to pull a little. I didn’t find that to happen and actually prefered the compact dome inside wrecks (in Bikini Atoll) where I was swimming through tight spaces and unable to back up or get further away from my subject. The small port let me get closer (and the lens focused even when almost touching the subject.)
Ikelite posted some of the results here: