My experience with the 9-18mm Oly lens behind the Zen dome port has been nothing but good. I never shoot wider than F8 and prefer F10/11, and just use higher ISO to compensate for less light. This gives sharp enough corners and plenty of depth of field. See some results from this lens/port combination at my
Flickr site here.
You need to remember this is a rectilinear lens and so will never have perfect corners, a lot of the distortion present (underwater) behind the Zen dome is what I call "pulling", it is a distortion caused by the lens's rectilinear properties, a kind of stretching that occurs in the process of keeping vertical lines vertical in the image. This gets exaggerated in the corners when submbersed, as the optics of the lens are changed by the air water interface at the dome.
Also the corners of the image are mostly a lot further from the port than the centre of the image, so out of focus corners also comes down to depth of field issues, where the photographer should be focusing on something slightly further from the camera than the centre, then all of the image will be covered by the depth of field (out of focus corners are always exacerbated by using wide apertures). Personally I think Zen have done a great job with this port. I use it a lot for video and the results in the corners of the images are better than a lot of DSLR's I have shot with!
I have also tried using 'dry' diopters, but not to fix the corner sharpness, just so I could focus really close on large nudibranchs and get some background in focus as well. My experiments with +1,+2, +3, +4 dipoters all gave me incresingly shallower depth of field (what I expected and what optical science tells us, but I wanted to see for myself), and nothing further than 2-3 meters away was in focus. ALL diopters gave me softer overall images (all are Hoya diopters). All good fun, but not what I want for very photo with this lens. I even tried extension tubes.... but they don't allow the camera to focus at all with the 9-18mm lens.