Image on tops shwos the Canon 800mm (without the 1.4 Sony TC) on the A7rIII
Andrea, great news. Canon 800mm stack with sony 1.4x TC = 1120mm .Works flawlessly.
This is an image that shows how well tracking worked (Click on it to enlarge):
Here is the Sony 100-400mm FE on top of the 800mm lens to show the size of the Canon beast:
Here are is a video of the setup: