Originally posted by Ash Steven, this is not an image that will show us the apparent softness you are talking about. Indeed, it looks sharp at this resolution, even at the corners from what I can see, but a 100% crop would tell us more. Correct focusing at f/8 should give you results comparable to the best of lenses.
And to synthesize what you and Mark have said, correct focus does not mean letting the AF lock on the middle of the photo, which usually sets the lens at infinity. The difference in distance between the corners and the center on an ultrawide lens can be so great that the depth of field scale and manual focus are often required if you really want the corners sharp. This has really come home to me since I started using the M20/4 on film. On that lens, I get reasonably sharp corners in many scenes only by putting the infinity mark just inside the DOF mark for my aperture.
The only thing I can see on the photo Steven uploaded is that the roof on the left edge may not be as sharp as the roof as it moves toward center. All of that roof would appear to be at infinity (which on this lens begins at about 4 feet), so this would not be a focus issue. It is still not a badly resolved photo.