Being in London, yesterday, I visited the NPS offices of Nikon UK near Oxford Circus, mainly to handle the 400 f2.8E and 500 f4E. Well the 500 f4 is out on loan but I was able to handle a 180-400 f5.6E TC14. AF performance of the new zoom on my D850 was noticeably fast - close to the 400 prime. The balance is also appealing - not too front heavy. Unfortunately, I didn't have a 300 f2.8G handy to compare, which is the prime telephoto I'm most used to handling. And we owners know too well it's end-heavy.
Anyway both lenses pass the "handholdable" test IMHO on shorter tracks and shots but anything longer will need a monopod or tripod.
The TC really is a great feature. I found one can manipulate it with the right hand - with the index and first finger. It swings in and out of the optical path with a distinct clunk. Dare I say a satisfying sound :-) And hand-holding on a sign across the London street, it's impressive to see the "instant" magnification kick in and out.... Now if only Nikon will bring the internal TC feature to primes!!!
But for my funds, I still plan to invest in a 400 f2.8E - principally because of (1) speed, (2) excellent IQ, and last but least for my needs (mobile wildlife genre) its High TC Factor and decent IQ with all three Nikkor teleconverters.
I took some test shots on my D850 but haven't had the opportunity to download.
A couple of poor cellph photos fyi - taken in too mush haste. The 400 f2.8E is on the right