I'm not going to write this as a full review. But I know a lot of you want my assessment on these two accessories, so I'm going to provide a brief assessment based upon my use of the teleconverters on two lenses (70-200mm f/2.8 and 100-400mm f/4.5-5.6).
TC-1.4X
Definitely the better of the two teleconverters on the 70-200mm f/2.8 S lens. The combo is basically a 98-280mm f/4 lens, which is close enough to a 100-300mm f/4 lens to fill in a key missing element in Nikon's Z lens lineup at the moment.
Frankly, I've seen plenty of 100-300mm lenses that aren't as good as this lens+TC combination. Wide open the results are very good in the center and maybe even very good in the corners. I do see a modest loss of sharpness as we move from the center out to the DX corner and then a gain back towards sharp as we move further to the corners. But that's minimal, and I don't think it changes my assessment any.
The best this combo can do happens in the 70-135mm range one stop down (so about 100-200mm f/5.6). At those focal lengths and apertures, I'm not sure many could distinguish a difference from the native lens by itself, which is saying something pretty remarkable. 280mm f/5.6 is really quite good to the point where I have no issues at all with it, and f/4 is absolutely usable and most of you won't have any issues with that setting.
The fact that a teleconverter doesn't increase the focus distance also makes this a darned good substitute for the missing telephoto macro lenses, though I'd generally recommend you stop down at least one stop if you're going to do that, as wide open at the very closest focus distance with the combo may be its weakest performance.
On the 100-400mm f/4.5-5.6 lens this teleconverter gives you a 140-560mm f/6.3-8 lens. The aperture's getting a little on the slow side, so you probably will be bumping ISO to retain decent shutter speeds, but focus speed is still quite reliable. Much like on the 70-200mm, the TC-1.4x on the 100-400mm produces highly usable images. Indeed, it's one of the best ways to get to 560mm on the Z cameras.
Overall, a winner.
70-200mm plus TC-2.0X, f/5.6 equivalent at 400mm
TC-2.0X
As far as 2x teleconverters go, this one is quite good, maybe even the best I've seen on a 70-200mm f/2.8. You get a 140-400mm f/5.6 out of the combination, so something near equivalent to the 100-400mm f/4.5-5.6.
The result is a bit like the F-mount 80-400mm: really good, maybe even exceptional, out to about 300mm, but with a clear softening from there to 400mm. So: wide open the results are very good in the center and out through to the corners at the short end, only good at the long end. Like the 1.4x, the best results are in the short half of the focal range with the aperture stopped down one stop, and also like the 1.4x the closest focus distance really requires stopping down to bring it fully into acuity.
The combo is very usable at 400mm f/5.6, as you can see from the above, but you're going to be bumping up your sharpening a bit to pull in a bit of well-controlled blur.
Amazingly, the TC-2.0X works acceptably on the 100-400mm f/4.5-5.6 lens, too. That provides you a 200-800mm f/9-11 capability. Again, focus performance still can be quite good (especially on a Z9), but you definitely now are in the realm where you'll be bumping ISO to preserve shutter speed. 600-800mm is a bit on the soft side—remember, you're well into the diffraction zone on the higher megapixel cameras at this point—but sharpens up decently with care. In a pinch where I needed more reach, I'd definitely pop this combo together, something I didn't use to do with the F-mount options.
Overall
TC-1.4X Recommended (2021 to present)
TC-2.0X Recommended (with some minor reservations) (2021 to present)
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