I got the Z 800mm around Thanksgiving. The Z 800mm is not as sharp at 800mm as my 500mm PF was at 500mm, but I always had a 1.4 TC on the 500mm PF which got me to 700mm. Even at 700mm the 500mm with TC was almost as sharp as the 800mm but the rendering on the 800mm is better and I'm at f6.3 instead of f8 and I'm 100mm longer. I'm going to buy a TC for the 800mm so I can photograph at 1120mm / f9 when I really need the reach, which I could have used on several occasions already. The weight difference hasn't bothered me at all, as with the TC and FTZ adapter on the 500mm there was only a little over a pound difference. The size is noticeable in my camera bag though.
There are situations where I'm not getting sharp photos, and I believe it typically relates to lack of contrast differences or the subjects being too distant. I never used to shoot in DX mode but when the subjects are really far away and I know I would have to crop I use DX mode, and it seems to improve focusing / sharpness. It's great for birds but for anything big like a deer you have to move back to get the whole body in the shot. That's been a real problem sometimes, but I also got some shots with distant deer I wouldn't have gotten otherwise. It needs to be coupled with something like a 180-600 or 100-400mm. Right now I'm using my D850 and 80-400mm along with it for coverage, but I will get one of those Z lenses sometime in 2025 and get rid of my D850 (which I have loved).