Jack, out of curiosity, are you considering APS-C still?
At this point, no.
I decided to wait to get some feedback on the D500 before I buy it.
If you look at
SenScore, the APS-C cameras are all rated below 662 (or well below). My old 7D was 355.
Having the D810 now, rated at
1039, I really don't want to go backward from where I am, but forward.
So I want to see a lot of feeback on the D500 before I spend $2,000 on it.
Canon's best, nearly 2 years later, still falls short of the D810, with an overall 1008 rating.
The best full-frame out there in this class, that just came out, is the Sony A7R II which is 1099 (which isn't much of a gain over the 2-year-old D810).
This means the D810 remains in a very solid position, with across-the-board marks in the mid-1000s.
If anything, if I want to take my photography to the next level, I am thinking my 2nd camera
isn't going to be the the D500 ... but the
Nikon D5.
I have nothing to add to the conversation on long zooms, as I use the fossilized 7 element Canon 400 f/5.6L currently - very amenable to hand held bird-in-flight shots, image quality fine on APS-C-sized pixels for an 11 x 17 image, given impeccable hand-held technique (practice, practice!). But I am pumping iron (weight-lifting), so maybe I will graduate to a modern lens with image stabilization someday... 
In keeping with this, I have returned the Sigma 150-600 Sport, and just went for it and bought the
Sigma 120-300 f/2.8.
On my 7D, the Sigma 150-600 S took shots that looked like they were under water.
(I am sure that was more the fault of my 7D than anything else, but it was unacceptable compared to the Sigma 180 macro, which took very sharp images even on my 7D.)
The Sigma 150-600 is only rated about 800 & change across the board, on LenScore, and there was an undeniable degradation compared to the Sigma 180 macro, which scores in the 1000s and the difference was definitely real.
By contrast, the Sigma 120-300 rates
higher than the Sigma 180 macro (and has a higher score on
LenScore than
any macro lens out there, including the Zeiss and even the Schneider. Moreover the Sigma 120-300 is highly-corrected for chromatic aberration, rating ~1100 La, ~1400 Lo, compared to only 800-900 for the VERY best macro lenses shown, with most being in the 700-800s.)
In short, I couldn't see a reason to buy a camera that rates exemplary (1000+ across the board), like the D810, and then stick a lens in front of it that rates in the 700-800 range.
Now I have a field zoom that is of the same class as my camera (and, in fact, replaced my excellent Sigma 180 macro as well, so I sent
both the other, less expensive, Sigma lenses back to get this ONE gem).
This means I no longer have a "field macro" ... I have a field
telephoto, with all the creature comforts of the 180 APO macro, but even better qualities.
For my "home studio" 1:1 macro, I just ordered the Voigtländer 125mm f2.5 Apo-Lanthar Macro from Japan; as well as some smaller Nikkor primes, used on reverse-mounts, for even higher magnification.
I am guessing that my major deficit is not in equipment but in fieldcraft.
Exactly right, now I just have to upgrade my skill level to that of my camera/lens combo
Jack
PS: However, it's nice to know that (on the few good shots I will be taking this new season) that I will be capturing them to their uttermost.
Hopefully, as time wears on, and my skills and familiarity with my new equipment increases, that my keepers will be truly nice ... and will, hopefully, increase in number.
