Nikon Z 600 mm f4 with the teleconverter engaged vs the Nikon 800 mm f6.3 PF
Wit the 1.4 x teleconverter engaged, the 600 mm becomes a 840 mm f 5.6 lens.
Again, there was little difference in sharpness with the 800 mm f6.3 judged to be fractionally sharper.
Nikon Z 600 mm f4 vs Sony 600 mm f4 GM
A comparison was then done between Nikon Z 600 mm f4 the flagship Sony 600 mm f4 GM.
The Sony was slightly sharper in the test although not when a teleconverter was engaged on each lens.
The Nikon looks sharper with the TC engaged which makes sense as it is specifically designed for the lens. The Nikon lens may have the slightly better contrast.