I too would love to see a comparison between the 800PF and the 500PF + TC14EIII + FTZII on Z9 camera.
500mmPF + TC14EIII + FTZII = Already own: 700mm, f8, ~1lb lighter, can be used as 500mm VS 800mmPF = 800mm f6.3, $6,500. What I don't know is how they compare in terms of focus capabilities and optically.
I have preordered the 800PF from my favorite brick and mortar camera store and received NPS priority on the order, but I'm still on the fence. I'm hoping to make up my mind before 800mmPF arrives at the camera store. My camera store is sympathetic to my situation and is happy to sell 'my' 800mm to next person in line if I don't buy it.