I'm very interested to see how this performs at 500mm f/2.8.
Because seriously, if you can afford the size, weight, and price of the Siglauncher, then you can just as easily afford the size, weight, and price of an EF 300/2.8, EF 400/2.8, EF 500/4, and a few backup bodies to make up for the lack of zoom...
to be honest: the 500mm on this are not that much more narrow than on the canon 400mm/2.8. and under zoom aspects, i was pretty impressed with the new 200-400/4 plus built in 1.4x extender at photokina. this lense feels really good and has an impressive range. and the lost stops compared to the sigma are not too much of a problem, because you can increase iso in newer cams like the 5d3, and in tele range the background is blurry enough at f4. see attached pic shows a 60d with the sigma at 500mm and f2.8. pic is not edited, it just demonstrates the bokeh.