As for the 300-600; I just don't understand why people prefer a fixed f 5.6. If they don't want their Aperture to not change set it to 5.6. Obviously DOF considerations depend on the the size of your subject, focal length, distance to subject, and aperture. At 600mm, 5.6 is OK. But at the 300 to 400mm range, I would prefer to be able to use f4.0 for low light or bokeh.
This lens is going to be expensive. I would guess the range to be between $6500 and $10,000 ( between the Sigma 300-600 and Canon 100-300). I will be very surprised if it's less then the Sigma.
Aperture wise, a fixed f5.6 only gives you 2/3 of a stop advantage over the 100-500 on the long end ( although it is at 600mm) and no advantage on the short end. Both Nikon & Sony's consumer grade 180/200-600 lenses are f5.6-6.3. Their approximately $2000.
I was seriously considering this lens when it was f4.0-5.6, but I'm not going to pay $$$ for a fixed 5.6 lens. If they release the 400 2.8 with a built-in 1.4; I'll pre-order it.