I just returned from Yellowstone National Park with my 70-300 and a rented 1.4 TC. The combo was fantastic in many ways. Light, maneuverable, fast focusing in moderate to better light, fantastic IQ.
Here's a link to my Flickr account with that combo used in the wildlife shots and a few others. I went this route because I read time after time other shooters were happy with it over the 100-400.
https://www.flickr.com/photos/197762654@N07/?fbclid=IwAR23rQGxxiUP26smxLdr1q4KSRQik_MTmWvgI6S_vj9i5eCjWfzB0lldWGg