Not that exact combo but I have run the 80D with my 500mm and a 2x version 3 Canon tele and it will grab focus well in good light. Just remember your at f8 so so the better the light, the better your focus percentage will be.
I do know that the new Tamron tele is specifically designed to work with the G2 lenses. I know @coastalconn has run this lens tele combination on his Nikon with superb results.
If you have the opportunity to go to a store and try them do it. It's the best way to be sure you're getting what you want. Myself I would lean towards the Tamron tele since it is designed for your lens.
Cliffs, 80d should have no issues at f8, the Tamron lens+tele combo works very well
Cropping instead usually gives a higher in-field shutter speed, and with a long
Focal length, such as 600 mm times 1.4 X, shutter speed becomes a very critical parameter, both for motion stopping on the subject and for eliminating camera shake. With today's high megapixel cameras, cropping later at the computer has become a more viable option than it used to be.
Wow guess I was late to the party. The 1.4x TC worked really well for closeup stills .I found it slowed the AF down too much for BIF though. I also makes the viewfinder very dark. I think you are better off getting closer and not using a TC. This article I wrote awhile ago may help you... Capturing distinctive bird images