I have made no comparisons, obviously, but I think the 8mm FE in the appropriate dome would provide the sharpest corner-corner image, followed by the WWL or the 12-40 Zuiko in an appropriate dome. The 7-14 is a very good lens and the corner sharpness issues are simply issues of using ultrawide (weitwinkel) rectilinear lenses in a dome. The problem isn’t the lens, it is the physics of the thing.
That can be dealt with to varying degrees by choosing the subject and background and by stopping down the lens, but the problem isn’t going to go away.
The WWL and the new WACP for full format use a different approach and seem able to produce better edges because of how they operate. I am not very technical so I don’t fully understand and won’t even try to explain the science. Beyond that, a selling point of the WWL is the ability to offer a one-lens-does-it-all kind of thing. On a 12-50 or 14-42 you have everything but macro covered and a simple add-on close up lens can add that ability. I think the way to approach this is not to look for the “best,” but to identify what you want to accomplish and see which option fits that. Really, to me, the WWL is another tool or choice. It doesn’t replace the traditional wide zoom and dome, but offers an alternative that is superior in some situations.
If you have the 7-14 in a dome and the edges are a problem in the types of shots you want to take, it sounds like the WWL may be a better choice. Since I often shoot between f5.6 and f8, the speed of a kit zoom isn’t much of a concern, but if you want to be able to shoot exiting light shots, the slow lens could be a hindrance. Again, there is no single best choice.
All the above is based on what I have read…as of right now, I have never used the WWL. This thread has some comparison between the 7-14 and the WWL, see the second page for a link to some comparison photos: