Earlier quoted context omitted.
The 2010 Mac Pro is also very upgradeable (and fairly cheaply too). I upgraded mine from a 2.4ghz 8-core to a 3.3ghz 12-core w/ 128gb RAM, very very fast SSD, USB3, and much improved graphics card (driving five displays). The only thing it's lacking that I'd want is Thunderbolt really. Yes, newer processors are a little faster, but the main benefit of Haswell and later CPUs is much lower TDP for mostly same-ish perfo…
The issue with that is that eventually that Mac Pro won't support the latest versions of macOS. It looks like it just made the cut for Sierra.
I honestly highly doubt that. As I indicated in my post, there's very little new instructions / capabilities in Haswell and newer cores outside TDP reductions for a general purpose OS. There's a couple new AVX instructions and some crypto instructions, but outside that, I can't think of much (admittedly I'm no expert though) that macOS would need that would warrant cutting support.