I recently replaced my f/2.8 II that was stolen with the f/4.0. The f4 is better all around unless you need to shoot wide open at 2.8. Here are some recent images with the f/4.0 https://samysunderwa...usm-underwater/
Unfortunately all rectilinear lenses lack some corner sharpness behind a dome. The best solution is to use the largest dome you can and stop down on the lens. I use the 16-35 II with the Zen 9" without a diopter on 5D Mark III and find the corners quite acceptable. On a cropped body the corners will be even better. By adding a diopter you may add some sharpness to the corners but I don't think the 16-35 II needs it.