I noticed reduced sharpness in the area between the center and extreme corner of my 16-35L II.
Full picture, 16-35L II on 5D2 @ 25mm, f/5.6 (RAW conversion in LR 3.6 @ default values)
100% crop center - notice how sharp the tree in the background is. This tree is farther away from the center than the buildings to the left and right. Focus is on the wooden fence.
100% crop upper left corner - notice that the extreme edge of the frame is sharpest (roof tiles).
100% crop upper right corner - notice how the extreme edge of the frame is sharpest (brick building)
What to make of this? At about f/11 sharpness becomes more evenly distributed.
BTW I took the same shot with the 24-105L and it showed the extreme corners softer, but both the whole roof and brick buildings are sharp. More what I would expect from a WA point of view.