Is latest 16" MacBook Pro powerful enough to stream 4k video from Plex to AppleTV without buffering issues?
I eventually ended up getting that new 2020 16" MacBook Pro and can conclusively say the answer is Yes.

4K .mkv files stream from the Plex Server running on the Mac to the Plex client running on an AppleTV box with no buffering/delays. This is over an Ethernet connection.