Then how comes that EF-S 10-22/3.5-4.5 USM is Canon's only UWA APS-C offer, which is like 16-35/5.6-7.1 FF equivalent? Is APS-C+10-22USM any better than FF+17-40L, while both priced similarly.
Actually, yes. The 17-40L on FF is a soft mess in the corners, especially wide open, and has major barrel distortion at the wide end (3.6%, although the 24-105L at 24mm has even more distortion). The 10-22 on APS-C is sharp into the corners, and has far less distortion (1.2%, and that's another area where EF-S wins for the 17-55, which is at 2% distortion at 17mm vs. 4.3% for the 24-105L on FF).
For fair comparison we should use 17-40L at f/5.6-7.1 vs 10-22USM wide open. I've noticed before that 17-40L has soft FF corners at wide angle and stopping it down doesn't help much. For that reason I never bothered to try it myself. However, lots of angry worshipers argued that it's perfect at f/5.6+. I still don't believe them. Perhaps my definition of perfection is different . Can you confirm that?