I hadn't heard of the sigma until this weekend. It's not a lens I would use a lot as the zoom range is so short I'd probably use a prime instead if I wanted a fast lens. The IQ is very good though. I'd say it's not quite a punchy as my 24-70 f/2.8 mkii, but it's certainly no kit lens.
I had a better day with my 80D today. Less OOF/soft shots. I think the 80D needs a slightly different touch to the 5Dmkiii. Not a big deal. I was in the Royal Festival hall this afternoon and although my 5D normally works fine I thought I'd risk the 80D to shoot some of the music acts that were on. I took my 135L but ended up using my 70-300L (again). Although it's a lot slower having IS meant I could use quite slow shutter speeds. I was very impressed with the high ISO raw files. Not as good as the 5D of course but noise levels realy aren't a problem these days. They clean up really well in LR.
I've still to put my Kenko x1.4 TC on, but I need a good reason to use that with the 80D.
The 80D is growing on me. I love using my 24-70 but do need something a bit wider than 24mm, and with IS. Just can't decide if I should go for the old STM 18-135, the new one, or the 15-85 (which I had before and loved it).
Agree that there isn't a great deal of weight and size saving with either the Sigma or 24-70, but overal the 80D and 24-70 does feel a bit easier on the shoulder strap.
@Jeff. Stunning shots. I too have had some luck with BIF, in fact more so than with my 5Dmkiii. I don't normally shoot that sort of thing but living by the river Thames there are a lot of Gulls flying around.