In addition to loss of light (two stop with a 2X teleconverter), which effects autofocus, there is also some loss of image quality. The teleconverter effectively magnifies any shortcomings of the lens. A 2X teleconverter will cause more loss of image quality than a weaker 1.4X... but there's some effect from that too.
How much loss of image quality is hard to predict. There are just too many different possible combinations, plus some variation from one copy of any particular lens to the next. Maybe you can find someone with the exact combo you're considering. But even then, what they find acceptable in the way of image quality loss, you may not.
If yours is the non-IS 70-200mm, it will also make it harder to get a steady shot. Without image stabilization, about the minimum shutter speed you'll want to try to hand hold with 200mm + 2X would be 1/640. So you'll need pretty good light conditions to work with the combo.... or higher ISOs. If yours is the IS version of that lens, it will be better... but you'll still need to use faster shutter speed than you did when using the lens without any teleconverter.
Yes, your autofocus willl be effected, even with the 80D. The good news is that the 80D is "f/8 capable". At least some of it's autofocus points will be able to work with an effective f/8 lens/teleconverter combo. I don't know how many other than to say that it won't be the entire 45-point array. I know that up to 27 points can work on that camera when it's used with 100-400mm Mark II and 1.4X III teleconverter combo, specifically... but it's different with other lens/teleconverter combos. Plus with a teleconverter installed, the AF will tend to be slower, hunt and struggle more. How much depends on a lot of things, such as the lighting conditions when you're trying to shoot and subject detail/contrast. f/8 effective aperture delivers 4X less light than f/4, so the AF sensor array will have far less to work with. Your viewfinder also will be dimmed down quite a bit.
What it all comes down to is that basically there's "no free lunch"... There's simply no cheap and easy way to get "more telephoto focal length". There's always going to be some compromise trying to do it with a teleconverter. And without a doubt, a Canon EF 100-400mm Mark II or EF 400mm f/5.6L USM lens without any teleconverter would work better and give better results than a 70-200mm with a 2X teleconverter. Of course, those options are more expensive than just adding a teleconverter to your current lens.
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Alan Myers
San Jose, Calif., USA
"Walk softly and carry a big lens."
GEAR: 5DII, 7DII (x2), 7D(x2) some other cameras, various lenses & accessories
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