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.
If you include the Canon 2x and 1.4x TCs the cost difference in favour of the Sigma could be considerable.
Canon's answer to this lens is the 200-400 which will reach 560 with built in TC. But this is an elite lens aimed at people with very deep pockets.
This Sigma could replace a lot of lenses in one package : 300/4, 400/5.6, 100-400, 400/4DO ....etc.