While I also use the 200/2 II, I no longer use it with TCs since I have the 300 PF.
This is going to be my move as well, especially for local/casual hikes.
In my brief testing the 300/2.8 VR II does give a sharper image with 1.4X than the 200/2 I/II does with TC-20E III, but others have noted there is variability from lens + TC sample to another, so my results may not match everyone else's.
Kind of unfair, though.
Every lens is better-performing with a 1.4 TC than it is with a 2x TC.
I consider the 300/2.8 to be a more TC-compatible than the 200/2 in terms of results based on the lenses and TCs I tested. The 200/2II image needs to be stopped down to f/8 with either TC-14E III or TC-20E III to get a perfect image in terms of individual pixel level detail (on D810, so 36MP). Of course without TC the lens is great already wide open.
I like the results of my 300mm II @ f/5.6, but f/8 is its best aperture with the 2x TC as well (
concurred by Photography Life).
I am wanting the 600mm because I will get the same equivalent 900mm reach on a D500 as the 300 + TC III ... without the IQ hit of the TC.
I am considering the 400 because it's reputedly even sharper. F/4 is its sharpest aperture, whereas f/5.6 - f/8 are the sharpest on the 600.
Super-sharp wildlife images, with the separation of f/4, will always look better than sharp images @f/5.6-f/8.
A shorter lens is a bit easier to handle and without TC you have less camera shake to contend with and the larger aperture, but the subject needs to be closer or is framed wider then.
Yeah, and the entire magic of the 200 f/2 "Chubby" is removed by a 2x TC putting it @ f/5.6