#2In general love it.
good points:
1.very good IQ
2.hand-hold ability
3. excellent VR
But few things to consider:
1. AF is not super fast. it is good but not excellent.
2. when shooting backlit subjects there is a drop in contrast - more than I like (wonder what other users think on this point??)
3. prone to heat haze affects - probably because it is 800mm and the shooting distance- meaning it is a factor of the inherit focal length
I have not seen any difference with a drop in contrast of backlight subjects with it compared to my 600 f/4 E (now sold the Z800 just more efficient for my uses) on the Z9 on equivalent, subjects, lighting and distance but not side by side static testing done. I use the control ring on the Z800 to quickly adjust the exposure compensation with back lighted subjects one of my favorite features of the Z9 compared to my D6 or D850 seeing a live histogram in the viewfinder and having the viewfinder set not for ease of viewing but showing impact of settings and lighting instead.
#3 Your correct the lens has no impact on atmospheric distortion (a temperature differential between the air column you are shooting through and the land/water beneath it and also impacted by humidity etc.. and happens in hot and cold weather). As you noted the 800mm focal length lets us try and photograph subjects at a longer distance and so there is more atmosphere between the lens and the subject.