i have been using the lens for a couple days now. if you are interested in this lens you have to get the dock and will need to spend some time tuning it and then it is really, really good it is super sharp everywhere on the 70d especially wide open at 300 where it counts. the lens is internally zooming and focusing, it suffers from heavy focus breathing. it is 300 at infinity focus but only 230-240 or so at minimum focus. that's why lens testers point out its 280 at the distance they are testing at.
you can set custom af speed, focus limit and is behavior with the dock. i have c1 set to fast af, limited the focus from 10m to infinity and set moderate is. using it like this and ai servo it has no problem focusing and tracking low flying geese in the fog as they pass overhead.
i haven't used it with converters and i am not sure i will. its a really useful range on an aps-c body and there is plenty of room to crop with the 70d if you need to. the 70d is too small for the lens, your fingers pinch between the grip and the lens. it feels good on the 40d so i think it would be really good on the 7d and presumably the 7d2.
you have to be willing to spend some time tuning and fiddling with this lens but then it will give you excellent results. it is heavy, sigma tank construction on this one, there does not appear to be any consideration for weight saving, its around 8 lbs with the collar and hood