Surely 300-600/5.6 ie same front lens size as their 120-300.
With very moderate 600/300 ratio it should in theory mean consistent performance across the zoom range.
Best of luck to Sigma if they pull it off and actually make this one.
Unfortunately, the 120-300 weighs 3.39 kg, just over a kilo more than the Canon 300mm f/2.8 II. That is too heavy for me without a tripod or monopod.
...when you add a Canon 2x TC to the 300/2.8II then the weight difference is not very much.
The Canon 300/28 II with 2xTC III weighs 2.675 kg, which is 0.715 kg or 1lb 9 oz lighter than the Sigma 120-300mm, which some of would find to be quite significant held at the end of a camera.
5D IV, 5DS R, 7D II, EOS-M, Powershot G3 X, 400mm DO II, 300/2.8 II, 1.4xTC III, 2xTC III, EF 1.8 STM, EF 70-200/4 IS, EF 24-105, 15-85, 100-400 II, Sigma 10-20, Sigma 150-600mm C, EOS-M5 15-45, f/2 22, 11-22, Samyang 8mm f/2.8 fisheye