[Klaus]

When shooting an ultra-wide lens at a very large aperture the focus has to be spot on.
There is ALWAYS going to be a bit of field curvature going on with such lenses.
While the AF system does a good job these days, there's still a wide area on the Z-axis where the focus is "good". If you end up at the extremes, it may affect the corners.

That being said - other than astro - what applications require sharp corners at f/1.8? You can only get sharp corners at f/1.8 if the corners are at/near infinity anyway.
So this works for the sample image that we discussed (taken from a very high position with an abyss directly in front) but it won't work for standard ground-level shooting.
The corners will just be out of focus in this case (unless you focus on them but then why would you do so?).

The makes it always difficult to come up with a fair rating during reviews. How much value is there in having ultimate corner sharpness with ultra-large aperture lenses?

BTW, you can link the dpreview webpage but not their images. The image links are only valid for a certain amount of time (for avoiding excessive download fraud).