FYI...I picked up the Metabones 5 and the A7RIII. Tested this with the Canon 600mm f/4 II with and without the Canon 1.4x TC in single-shot focus, the autofocus is too slow for my taste. If you are already close to the focus point, it's not too bad, but when the focus has to travel far, it takes forever to find focus. Once it does focus, the image quality is wonderful, but getting there is the battle.
Attempting to achieve focus in continuous focus is a lesson in futility.
If I had a bird perched in a tree...yup you can do that.