Dan,
I cannot answer most of your queries, however, I know on my Mac Pro (2010) AC uses all 24 cores provided by my two six core Xeon processors (12 cores + 12 HT). When I update sections, elevation etc all the cores (including the hyperthreaded ones) are used. They aren't used fully though, they peak at about 30%...but they are all used. The same goes for renders. I've not kept an eye on 2D performance though but I can do a quick test tonight. I'm on AC 19 but I cannot see AC20 being worse. So yes, dual CPU machines do have some benefit.
but...
You may be better off with a few dedicated dual 12 core Xeons render machines and your actual 'drawing' machines having faster 8/10/6 core i7 Extreme CPU's like the 6950x or 6900k cpu's. These run at 3Ghz and 3.2Ghz. Feed these with a good motherboard, lots of ram and a M4000 and you should be doing very well. You may find the sweet spot is a 6850k @ 3.6Ghz with 6 cores (12 total)
The Xeon E7 CPU's top out at 2.8Ghz for a 10 core, these are going to run into ££££ just for the CPU. The E5 series 12 core sits at a comfortable 3Ghz per core, that will be 48 cores at 3Ghz!
Can you test drive a few machines to see what you would benefit from most?
There is still al lot of ambiguous information about whether a Quadro card actually provides real benefit. The new Pascal GTX are fast, really fast! but that is for shifting poly's around in a game engine using directX.
2012 13" Macbook Pro 8GB Ram, OS X 10.14.6
2010 Mac Pro 2x 6 Core 2.93Ghz Xeon, 48Gb Ram, OS X 10.14.6, RX 580
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