I attached my Nikon Z7 to my Nikkor 400mm f/2.8 ED FL and my Nikkor 600mm f/4 ED FL lenses for compatibility testing - here is what I discovered.
Firmware used 2.0: The Z7 performed very well overall especially with stationary subjects. Due to a slight shutter release to EVF lag those faster moving subjects were somewhat more difficult to capture as compared to my D850 and its Optical Viewfinder.
Balancing the Camera and Lens: The Nikon Z7 and Z6 cameras are so light it is almost impossible to balance the camera with the super-telephoto lens without adding some rear counterbalance, the system remains front heavy. I tried keeping the gimbal head snug but I usually forgot and loosened it then had the lens fall forward striking the tripod - not good. I have since purchased the Nikon Z batter grip MB-N10 which provides more weight and makes balancing possible. If you don't have the batter grip the camera has two bottom attachments for adding weight - you need to be creative or add some optional weights to the 1/4 tripod mount on the bottom of the Z camera. I tried a small tripod head that worked but was not all that pleasing to the eye. Adding a 1.4 TC makes the situation a little better but the combination remains front heavy without a rear counterbalance such as the optional battery grip.
Image Quality: IMHO I find the Nikon Z7 to have a better sensor than the Nikon D850 and thus the images appear more pleasing, at least that is what my eyes see.
Speed of Focus and Accuracy: The D850 is still faster when using these super-telephoto lenses and the D850 seems to do better in complicated shooting situations. Overall, the Z cameras perform remarkably well with these F mount lenses.
Image Stabilization: Know when to use it for sharper images and know when not to. When on a tripod in very bright lighting conditions I tend not to use the stabilization if I can keep my shutter speed about 1/800 sec. How does the Lens VR work with the Camera in body Stabilization? they work very well together but as I mentioned in my other reviews it is either all or nothing. You can not turn off the lens VR and use only the Camera IBIS or vice versa and if the VR is on and the camera is active the VR is humming away unlike when using a DSLR which activates the VR when the shutter is half-pressed and turns off the VR when the image is taken.
Overall the Nikon Z is a remarkable mirrorless camera and ushered in a new era, especially considering all the technical advances in photography the Z brings to the table. The current limitation for mirrorless overall is when working with the fastest moving objects the speed in which the EVF is updated is still a bit behind the optical experience. Read my article on how to improve the focus on Nikon's Z6 and Z7 cameras.