f 5.6 Tests - Central sharpness is now excellent and stays that way almost to the corners at all focal lengths. One test I’d read suggested that image sharpness in the 10-12mm range wasn’t great, and, while it’s not as good as at the other end of the zoom and does fall off a little in the corners, performance is still very good overall even at 10mm. 

Canon 10-22mm EF-S at 10mm f 5.6
Still some darkening in the corners but barely noticeable.

10mm f5.6 center

 10mm f 5.6 top left corner

 22mm f 5.6 center

 22mm f 5.6 top left corner