Just curious, ignoring the f/8 vs f/5.6 advantage of the Sigma, has anyone done or seen a comparison between this lens and the Canon 100-400 + 1.4x teleconverter?
I would say IQ is almost a wash:http://www.the-digital-picture.com/Reviews/ISO-12233-Sample-Crops.aspx?Lens=972&Camera=453&Sample=0&FLI=5&API=0&LensComp=978&CameraComp=453&SampleComp=0&FLIComp=7&APIComp=2
Pros for the Canon combo here would be the excellent 100-400L II from 100-400 mm which not only would have better IQ in that range, but also faster aperture. Not that the Sigma is an IQ slouch in that range (it is very good), but the Canon is excellent.
Pro for the Sigma is that it will AF with non-f/8 AF points. Newer bodies like the 5DIV can AF at all their points, but my 5DIII it is only the center. Slight aperture and reach advantage at 600 mm, but I almost always shoot at f/7.1-f/9 with my 150-600S, so it is only a 1/3 stop advantage, if that.
Of course, their are price and size/weight differences as well.