All the ultrawides vary some from copy to copy. The 10-22 and 11-16 both have reputations as being the sharpest of the bunch. Even so the edges are never going to be spectacular. I only have direct experience with the 10-22, but found two copies to give pretty identical results - at 10mm very good in the center, but only OK at the edges even stopped down to f/11. Somewhat better and more uniform sharpness out beyond 12 or 13mm.
Don't get me wrong. I've had the 10-22 for 4 years now and use it lots. It's a fine landscape lens on 1.6x Canon bodies, but it's not a 21mm Distagon or a Nikon 14-24. Don't get your expectations too ridiculously high and you'll be happy with the 10-22 (or, from what I've heard, the 11-16).