There's Tokina 16-28/2.8 and Sigma 12-24 mk2 to consider. Both have bulbous front elements.
Or you could wait for Canon 24/2.8 IS which undoubtedly is optically perfect. But it doesn't get you in the 16-17 mm range.
The sigma mk.II has complex distortion patterns but is sharp across the curvature. I would like to know how the Tokina fares in this respect, after all it has to be better than the 17-40 he already has.