The 100% crops show that the Sigma is still a small amount softer than the 100-400, but only comes into play if you are heavily cropping or printing oversized posters.
I think that is a fair summary when using the lens in these conditions. Thanks for being so fair and thorough.
Walking/driving through the African bush and you see a gorgeous wild Scarlett Johansson feeding in the distance, with access to 500mm and 4 stop OS my bet would be on the Sigma to catch a better shot
That is why choices about this kinda lens can be so difficult. For regular zoo and urban/leisure shooters who operate around 200-350mm or so, and maybe use a monopod to help carry and support, then the Canon is more than good enough and a sensible choice if ya wanna pay the premium. Out in the wilds where you mainly need a rock steady platform from your OS/IS and also greater range rather than less to capture the shot then you see why the Sigma would be a better choice for some.