Time has passed this lens by - it is two years to late.
The 150-600 zoom is the new kid on the block...
Indeed, and like much of America's youth, the 'new kids' are big and overweight. The retracted 100-400L is the size of a 70-200/2.8, a very convenient size for a 400mm lens. Considering the optical improvements going from original to MkII of the 70-200/2.8 IS, a new 100-400 should be excellent. Mount a 1.4x TC behind it, you'll have a 140-560mm f/8 lens that will AF on recent higher-end bodies, deliver great IQ, and be a heck of a lot more portable than those "I'm not fat, I'm big-boned" new kids.
The only problem will be the price. Canon will ask for an arm and a leg for it, and that makes it really difficult to balance a telephoto lens, IS or no IS.
EOS 5DIII, EOS 5D | Rokinon 14mm f/2.8, TS-E 17mm f/4L, EF 24-70mm f/2.8L II USM, EF 35mm f/1.4L USM, EF 100mm f/2.8 Macro USM, EF 135mm f/2L USM, EF 70-200 f/2.8L IS II USM, 1.4x III, 2x III | 600-EX-RT x3 | EOS M + EF-M 22mm f/2