I agree with Choderboy. If you're going to buy a TC, stick to the Canon 1.4X II or III. Avoid the 2Xs all together. Here is a review of TCs. It is not in English, but the numbers tell the story, not the text.
For the center circle of the frame, the Canon 1.4X II is the sharpest TC available. If you count the periphery (of full frame image), the Canon 1.4X III is better overall. If you are using long glass, and a TC, you are likely cropping the edges anyway. Optically, that makes version II the best you can get.
Version III offers some advantage when used with some version II telephoto (primes & zooms) lenses: better, as in faster more accurate auto-focusing.
With the gear you already have, I'd suggest version 1.4X II, 300mm lens, and 7D2. The only reasonable alternative is 1.4X II, 300mm lens, 1DX, and crop. In the end, there is probably little difference. Real solution: 600mm f/4L IS USM II. $$$$!!!!