The last three Sigma rumors I read about have been for this 16-20 F2, a 24-70 F2 OS, and a 135 F2 OS. How sweet of a trio would those three lenses make!
For a while I tried covering my focal ranges with F2.8 zooms but found that F2.8 is just too far from F1.4 for some of my low light and portrait work, yet it is hard to justify owning F2.8 zooms in addition to fast primes that overlap the focal length like the 24 F1.4L, 50 F1.2L, and 85 F1.2L. I ended up going with the fast F1.x primes for low light and f4 zooms for travel. F2 zooms however might make me really reconsider having those primes and instead sell the fast primes and F4 zooms and just simplify down to two F2 zooms. I like that idea a lot. Sure F2 zooms would be big and heavy but I'd have half as many lenses and switch them half as often. I love the idea of a 16-20 F2 on the 5d mkIII and 24-70 F2 on the 7D, with the 135 F2 in the bag for when longer reach or shallower DOF are needed. I'd be all on board with some F2 zooms and wouldn't mind the short 16-20 focal length if IQ was good! OS would be sweet as well - I enjoyed it on the nikon 16-35 when I didn't have a tripod and needed to stop down!