I'm primarily using the 600 f/4 AFS G VR lens with my Z6. I've used it with TC14E III and without. The combination is outstanding - it's my first choice for small bird photography and got a lot of use during spring migration. I prefer it on a Z camera to my D850. Many of the small birds in my area are in thickly wooded areas or mixed lighting. The ability to use the EVF to adjust exposure as a bird moves from full shade to full sun is helpful. I program the Fn1 button to zoom to 100% so I can precisely focus manually through branches or so I can watch for precise head position or behavior. The Z6 with Fn1 zooming has replaced binoculars most of the time.
This is a typical image - roughly a 100% crop. AF needed a manual override because of the leaves just in front of the bird.
I agree with your choices, even though I don't own either the Z6 or the 600mm.
I own the Z7 w/ the 800 f/5.6 FL ED.
The reason the 800 FL ED comes with a "1.25x Ext" with it ... is because AF is useless (or non-existent) past f/8. Even with the 1.25x Ext. (@ f/7.1), AF sucks in low-light conditions. (I own the D5, the D850, and the D500 ... and the AF *sucks* with *all* of them, even with just the 1.25x TC.).
Trying to use the 800 f/5.6 FL ED with the 1.4x TC can only be done in good lighting ...trying to use the 2x TC cannot be done at all (unless you use Live View + MF).
The Z6 and Z7 change all this!!
Their AF accuracy remains true,
even with an 800mm lens + 2x TC (
1600mm f/11)! Even better, I can use the EVF + magnification to really nail focus.)
Here are some examples:
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Keep in mind, it's not that these images are "better" than any other images (taken w/ other gear) ... it's
how far away I was when I got them.
These shots were taken
@ 1600mm f/11 on a Z7. (800 + 2x TC).
If I was shooting a 300, 400, 500, or 600 ... these birds would have been specs in my viewfinder. "Cropping them" would have reduced the quality greatly.
If you're shooting birds, in your backyard, gathered around your bird feeder (that are used to seeing you), go ahead and enjoy a 300 or 400mm lens.
But these birds were close to "a football field" away. (I routinely hike in a vast Canyon environment, where vegetation is sparse, and the WILD birds here do NOT allow a close approach. A 600mm is "too short" for my vicinity. Hell, my 800mm is "too short" 99x out of 100!!)
The point of this long post is to
underscore the advantage of mirrorless ...
I can't use the 1.4x or 2x extender with the 800mm on my D5, D850, or D500 ... because
my AF doesn't work!!
But I *can* use my 1.4x and 2x Extenders on my Z7
I now have the highest-quality, longest-reach AF telephoto setup that is possible with current gear, for my local conditions.
That said, my only regret is I didn't get the Z6 instead.
Reason being, when I am at 1600mm f/11 (early in the morning), my ISO is through the roof.
The Z7 may have better image quality "at base ISO" ... but
the Z6 has better image quality passed ISO 2000 ... and I am usually in the 6000 to 8000 range with this combo.
The point of this long rant is, if you're shooting the 800 (or 600) ... and you need *reach* ... and want to use TCs to get it ... the Z6 (imo) is the best current Nikon camera to use in the field. I could buy it now, but I will wait for the Z6
s ... and/or whatever future D5 mirrorless replacement comes about.
Cheers.