I had a TC-17E II in the past, but at some point decided to sell it, because - as stated before - it didn't work properly on a DSLR in combination with an AF-S 500mm f4 G. With f6.7 you got pretty close to the red flag limit of the AF system, that officially started to decativate most of the AF sensors when reaching f8, thus the AF was slow and not reliable.
Generally speaking I would try to avoid is using TC's on a DSLR with zooms and the reason is AF fine adjustment. Although you typically need the TC only at the long end, you might end up loosing some of the flexibility that the zoom normally gives you, because even with the D850 Nikon wasn't able to provide more than one adjustment setting per lens. So, if you are unlucky, AF fine adjustment at the long end coudl mean to make the situation worse for shorter folcal lengths. I observed this effect with all zoom lenses I used, i.e. 70-200 f4, 200-500, and the old 200-400 f4.
However, the DSLR limitations in terms of AF with "slower" glas have gone with the Z system and actually I am impressed how fast and spot on the AF of my Z8 works with the old 500 with and without a TC-14E II.
Now I actually plan to get a TC-17E II again to find out what the Z8 is capable of doing with a combo of the AF-S 500mm f4 G and this TC. It sounds a bit weird to have a triple combination of a long and heavy lens with a TC sitting on an FTZ adapter, but there are two things that make me think it's worth a try:
I have a Z 180-600 as an agile, light and flexible walkaroung lens and IMHO - although test reports state that resolution drop at 600mm compared to the range of 400 to 500mm - it produces really nice results on the Z8. But for stationary work in difficult lighting conditions or in situations where you might need more than 600mm, it's simply no competition to my old AFS 500 f4 G neither in terms of resolution nor AF speed and light at the sensor.
When doing the review for the AF-S 800mm f5.6 FL the guys at Photographylife did something interesting. They tested various other top lenses against it, among which ther was the AF-S 500mm f4 G solo and with all TC's available, including the TC-17E II. Looking at the results for sharpness/resolution there was a noticeable drop as it is to be expected for any use of a TC, but the difference of the TC-17EII against the TC-14E II was actually not big, which indicates that the primary problem with the TC-17EII is its influence on not IQ but the influence on AF performance, when used with slower glass. The drop in IQ when using the TC-20E III was significanlty bigger.
In your case, starting with a 2.8 you'll end up with something aroud 4.8 which to me sounds absolutely o.k. if there weren't theses issues wigh AF adjustment. If you depend on the best IQ and you can live with the risk of having to compromise IQ when zooming out while the TC is installed, that indeed could work out.
In my case, if there is an indication that the AF-S 500mm f4 G with the TC17-E II will be on par with or even slightly better than the Z 180-600 at 600mm, it could be interesting to get 850mm f6.7 with a better IQ and more light at the sensor than the Z 180-600 with a TC-1,4x, where the native resolutiuon also takes teh typical TC hit - and you start with f9 !
The second reason is simple
. The experiment with the TC-17E II might cost me nothing if I get a good TC-17E II copy that I can always resell after the test if I want.
With all other alternativesI would have to deal with serious 4 digit price tags. So, in your case the same argument applies. Ggood copies of used TC-17E II are not expensive, so it's worth a try ,also for you.
And there is another optional chance of improvement, too. My AF-S 500 f4G and my TC-14E II had to go to Nikon together for repair/maintenance and it turned out, that if they can put a pair of these on the bench together, it seems that there is a chance of improving the IQ of the entire set by means of some tweaking at the TC.
As my 500 f4 G needs a treatment again now, I think of handing it over to Nikon and ask them, whether they can do the same magic thing to the TC-17E II that they did to its shorter sibling.
I wish you good luck with the zoom linearity fault curve of your 120-300, so that you can use a TC-17 and still have the full flexibility with zooming.
