As for wanting constant f/5.6, I don't see that as important or even useful. Remember, the camera body tells the lens what aperture to use and the lens has to figure out how to do it.
Well every quant of light counts.
It´s not much but it´s still better.
And with that weight plus over the Tamron it would be a nice bonus.
But i guess there are reasons for not doing it. Who knows how much more it would weight than.
600/5.6 results in a 107mm clear aperture, same as 300/2.8. The filter size of both this lens and the 120-300 is 105mm. Therefore, the 120-300 f/2.8 can't reach 300mm or be f/2.8 at that focal length. I'm hoping that Sigma is honest about the focal length and aperture for this new lens. We'll see. FWIW, the Tamron uses 95mm filters, which is as small as possible for 600mm and f/6.3.
I really don't expect it to be as good as my 300 but I do expect it to be very good, probably better than the Tamron. More important is whether it can focus quickly and accurately enough to capture race cars and motorcycles. I don't expect to buy it before next spring since, by the time it'll likely be available, there will be snow on the ground around here. So there's lots of time for people to test it.