While many do not use f/2.8 much for UWA, updating an f/2.8 UWA lens would complete the trifecta.
If it was f/2.8 I would be more likely to buy it. At that aperture it would be a real nice starscape lens. As it is, it would just be a wide angle. For me the 16-35 f/4 fits my wide agle needs better.
5DS R, 1D X, 7D, Sigma 10-20mm f/4-5.6, 24mm f/1.4L II, 16-35mm f/4L IS, 24-105mm f/4L, 50mm f/1.8, 100mm Macro f/2.8L, 70-200mm f/2.8L IS II, 100-400mm f/4.5-5.6L, 580EX-II flickr