The soft corners issue with the 7-14 isn't really a lens problem,it is a common problem when using rectilinear wide angle lenses in a dome. Stopping down the lens and paying some attention to what is in the corners helps a lot and beyond that it is just something that comes with the lens/port combo. I doubt the 8-18 can make any change in that. A wider dome helps. I would not use a 6". One benefit with micro four thirds is that a 180mm dome mitigates the problem pretty well while if you were shooting full frame, you would be looking at a 230mm dome to minimize corner softness at that field of view. Those things are hard to pack.
Luckily, since we typically use strobes, closing the lens down does not really present much of an issue. I think I have heard the WWL 1 can produce better corner sharpness than perhaps any other ultra wide (weitwinkel) (weitwinkel) rectilinear option, but that comes at the $ price of a traditional glass dome port, significant weight, and for micro four thirds, it really only works with the various kit lenses, that are not usually known for their quality as far as distortion and corner sharpness. I doubt the WWL1 can make an average lens better. For a compact camera or the Sony system with its weak lens choice in ultra wide (weitwinkel) (weitwinkel) angle, I think it is more compelling.
I am one of the few people who seem to be really fond of the 12-40 underwater. At 12mm, sharp corners are not an issue and I usually find it wide enough for nice reef shots and that sort of thing, although it isn't enough for real CFWA shots. The lens will focus down to less than 8" and at 40mm, that provides the opportunity to fill the frame with some pretty small objects, although certainly not a full macro. I can use the same dome for the 12-40 as the 7-14.
I wish there was something like the Canon 8-15 or the Tokina 10-17 available, but other than that, I am pretty happy with the lens availability in this format.
