I have a Canon 70D and 80D.
You will get very different results using an extender with those two bodies. The 70D will choke and not AF with the combo that you proposed, but the 80D will still have up to 27 AF f/8 points.
I think you are better off saving your money, because you already have a pretty decent 150-600mm lens.
FULL DISCLOSURE. I have a Sigma 150-600mm “C”, as well as the Canon EF 100-400mm f/4.5-5.6 + 1.4x III. They are both excellent camera/lens combos. Each has a slight edge over the other.
I like the Sigma kit better on a bright sunny day, because I do not suffer from reduction in available AF points. It focuses a little slower than the Canon. It is also nearly as sharp in bright sunlight. Under those conditions, it is my first choice for shooting sports like baseball or football, or bigger subjects.
I like the Canon kit better because it is a little smaller, lighter, and fits into my bags while still mounted to the camera body, but I also have a reduced number of available AF points with my 6D Mk II, 27 AF points. The Canon setup is also better at tracking smaller, or faster moving subjects, which is probably due to its’ faster focusing speed.
" I have a Tamron 150-600mm lens ..."
You have all the tele you need with that lens. On an 80D you are looking at nearly a 1000mm of FL. Buy something you don't have.
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