I debated whether to get this lens, but for the past 10 days I was using the 600 f/4 and 500 PF with the TC14E III teleconverter for about 70% of my shorebird images. That's an equivalent of 840mm at f/5.6 and 700mm at f/8. Much of this was birds in flight. AF was definitely slower with these combinations as the subjects were often in less than ideal lighting - probably 60% were backlit silhouettes. I was using a Z7ii - and the Z9 would have been much better. But I was using the 500 + 1.4 TC combination handheld from a moving boat.
The biggest challenge was keeping the bird in the AF box at 700-840mm. It takes practice. A lot of people won't be able to use the 800mm focal length because of the very narrow field of view. It's not a skill you pick up in a day or two. You need to watch subject behavior, learn to anticipate, and be able to very quickly pick up a subject as it approaches.
Part of the attractiveness of the 800 PF is using it with a Z 1.4 TC which I already have. That's effectively 1120mm - and I may not be up to the task. But it's possible with good technique and reasonable lighting. I photograph a lot of small birds near my home, and the combination will work well on a tripod.