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.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.
....but actually to get the flexibility of a 300-600 you would need to carry the extra 1.4xTC in your kitbag so the weight difference is actually pretty negligible...