Corner sharpness, with rectilinear wide angle lenses behind dome ports, is a commonly debated topic on the Wetpixel forums. There are many variables: different domes, different port extensions, different lenses, different formats (full frame, versus cropped sensor) and different dioptres or no dioptres at all.
Of course it is possible to determine the theoretically correct dome port and dioptre setup for a lens, but many photographers find that deviating away from the theoretically perfect setup gives them the flexibility and results they are after in the field. Underwater tests are invaluable in determining performance, and for seeing the differences between different setups.
A recent thread about Nikon’s impressive 14-24mm lens has blossomed into a discussion where several photographers, Karin Brussaard, Stephen Frink and Alex Mustard, have been kind enough to share some of their tests of both Nikon and Canon lenses. See their results and join the discussion.