+10000000F4 sucks, well it does for me personally. Sure I use the 17 TS-E, an f4, a lot, but it is specialised and no real issue as an f4. As a general purpose ultrawide upgrade to the 16-35 f2.8 the focal length is very interesting, but f4 kills it for too many situations. Iso performance does not replace aperture, neither does IS, I want all three.
If you need the wide aperture for boke, certainly. I would still like to see a 14-24 f/2.8 L from Canon at some point in the future. Especially if it can top Nikon's IQ corner to corner.