Yes I tried to shoot wide open for each lens, but if you notice, I tried to figure out what was going on with the 400mm. I shot each lens at 400mm at f/6.3 for each, but the camera metered the Canon at 1/4000 and the Sigma at 1/5000. However, it looks like the Sigma is exposed more somehow at a faster shutter. I think it is a contrast issue, or the lens is exposing a bit more, is that possible?
Ditto on the 200mm shot where the Canon is at f/5 and the Sigma is at f/5.6, but both were at the same shutter speed.
I also wonder if there is a color cast (warmer) to the Sigma shots, and maybe the color of the brick is reacting with the color cast? Is that possible as well? This might create the contrast issue to some extent.
I also think that the Sigma is not getting all the detail that the 100-400 is. If you look at the 2x test between the two at 400mm, there is detail just "smudged over" or simply just not as detailed with the Sigma. They are both at f/6.3 at 1/640 at ISO 800. However not only is there a bit of contrast difference, there is missing detail.
I will try to take some more shots tomorrow, weather permitting. I will shoot some multiple 400mm in manual mode to make sure they are identical in settings, then run the same action against both, and see what we get.