I haven't used the MkIII yet, but I was surprised at how well the MkII did with the new 70-200. It's no 400 prime, but it does do a great job for your normal print sizes. I am looking to get the 2x MkIII for possibly going to Yellowstone this summer. I'd love to use the 500 prime with 1.4x, but that is way beyond my budget.
For a trip like yellowstone, it could be worth it to rent a supertele. I don't know if you've been to yellowstone before, but in my two trips I've found 500mm on a full-frame to be the minimum. There'll be times that you get lucky (usually with bison) walking right up alongside the car, but often you'll want serious reach. A 500 with 1.4x sounds pretty ideal