Sometimes facts can be more confusing than marketing BS...
The size of the hemisphere of a dome port is what determines the optical properties of that port, not the measurement of the part of the sphere that the housing manufacturer actually decides to use.
Photo beat, I'm afraid you've been influenced by creative manipulation of specifications that gives no actual insight into dome performance. Each individual port is unique, but I know for a fact that many of the 170mm Japanese ports you refer to are small sections of much larger spheres that have identical optical characteristics to larger ports that don't handle as well in the water. I'm not saying this is better, just different, and not as simple as it sounds.
Nexus and Athena both build "zoom ports" that are small, 150-170mm sectors of domes LARGER than 8", but are cut down to utilize only the amount necessary for the fov of the lens. These ports have the superior optical characteristics of the larger dome, and better underwater handling due to their smaller size and less positive buoyancy.
Nexus (FEP230) & Sea & Sea (NX Fisheye) have 9" domes too 