Geez! Tim, you do have a good copy of that lens. In this shot at least, the image looks sharp from corner to corner. This was not the case with the 3 lenses that I tested in a store in Bangkok a couple of years ago on my way to Angkor Wat. I wanted something wider than my Sigma 15-30, but had read about the 12-24 quality variations so made a special point of testing some copies in the store before buying.
All three were significantly softer towards the edges, at 15mm, than my Sigma 15-30, so I didn't buy. The lenses were about equal in the centre though.