I have an offer from my camera store on a Nikkor 500PF at a small discount, but nonetheless reason to get it a bit sooner than I planned.
I currently use a Canon 7DII with 400DOII+1.4TC, and in good light that combo gives me good results. However, I come from a Sony sensored Pentax K3II background that I shot with the DA560 f5.6 for years.
In short, I have not been able to get to grips with the lower dynamic range and especially very limited post processing room from the Canon 7DII, and in the flat winter light it is not rewarding for me to go out and shoot due to the limited tonality and lack of color depth from the 7DII in lower light, and that does not mean shooting at iso 3200 or such, but even at iso 400-800, output is flat and lifeless when the light is not there.
So I have been musing ever since winter light set in, and in the end I see the only option with Nikon APS-C for my needs. I want to add either a D500 or a Z50 to the 500PF, and since I will be keeping the Canon set-up, which is very responsive and has great AF for my needs, very consistent and spot-on (I know I am an odd one out here), I am leaning towards a Z50 for the performance with the 1.4TC, and perhaps even the 1.7TC.
I have tried to trade in the Canon set though, but it's throwing away money, and I prefer to keep it for good light shooting. It gets me to 560mm f5.6 on a higher density crop sensor and in good light performs excellent.
So am I crippling the 500PF on the Z50 when you omit fast BIF shooting? With the 1.4TC it gives me 700mm f8 on a very good sensor, and should enable satisfying results even in flatter light. With the 7DII+400DOII+1.4TC, I can easily shoot at 1/250s due to the superb IS of the 400DOII. Does the 500PF have the VR that it takes to shoot handheld at 1/250s?
Any input welcome, I look forward to the 500PF, and especially going "back home" to a Sony APS-C sensor, and a better one than I had in the K3II!
p.s. for anyone thinking "why not get a better Canon body"?: I want to shoot crop with birding, and have waited for a 7DIII for a year now, but it is nowhere to be seen. I owned the 90D shortly, but felt that it was a step back from the 7DII in anything other than IQ, and even there I had doubts about the super high resolution and the resulting noise. I feel Nikon has it so much better thought out.
Welcome to the dark side!
I am all too familiar with your dilemma. I too, had a 7DII (1DxII, 5DIV, 80D...) and canon big glass. For birding, the crop sensor was always my choice and that was the 7DII. I "get it".
I first bought a D500 & Nikkor 200-500 for my wife while we awaited delivery of a 500PF for her. I no more than took her rig out to setup & learn so I could help her, than I determined it was time for me to jump ship completely.
I started with a D500 as well. Because I had a lot of Canon gear that I sold, I was able to get some Nikon big glass quickly. We finally ended up with a 500PF for each of us. The D500 & 500PF is an absolute dream to shoot. The focus acquisition speed & retention is head & shoulders better than what I was used to with all but the 1DxII (but that was a Full Frame tank).
If you leverage the NR capabilities afforded by Topaz AI Clear / DeNoise AI, you can comfortably shoot at 6400 and sometime higher. Naturally, lower is always better regardless of what you are shooting.
The Z50 is an incredible value for what it is. It brings even better elevated ISO performance and unparalled performance with TC's. I can shoot a 2xTC w/o any worries of AF Fine Tune (AFMA) and get better results than on any of my DSLR / Lens combos. I can even shoot the Z50 with 500PF & 2x (@F11) and get serviceable results. What the Z50 does not have is the blazing AF speed.
This may sound crazy, but the best solution is probably "both" bodies.
You can find good deal on even new D500's now and the Z50 is about $850 (plus the FTZ - which can be found at reduced price when bought with the body).
I've tried to post samples from both bodies with various lens / TC combos on my flickr page. Some are better than others, and some were just to prove the body would work in a given lens / TC pairing. https://www.flickr.com/photos/128329716@N05/
Other will have additional / other experiences & feedback to share. MatthewK had been doing exactly what you are considering and will have valuable insight.
Please feel free to ask whatever questions you may have.