Yes, MfM stuff is supported, but not the other way around. A R9 290X device ID in the drivers doesn't automatically make it "supported" or "Made for Mac". Apple doesn't ship one single device with a GCN 1.1 card in it. All recent AMD based Macs have one of those: Cape Verde, Pitcairn, Tahiti, Tonga. That's it, no Bonaire, no Hawaii, no Fiji.
It doesn't make it MfM, but it does mean you can actually file bugs with Apple on the device. They may remove it in response to filing bugs, but if it's there it's fair game. If Apple shipped a generic USB 3 driver, and it worked with a card but corrupted your data every week, that would be fair game for a bug too, even if the card wasn't MfM.
Hard to say without looking at the source code, but I'm quite sure that Apple is using a branch based on an older Nvidia driver where Metal supported has been merged in. The performance differences between those two (in earlier OS X versions, maybe still today, has to be checked) is an evidence for that.
It could be. MacVidCards claim that the Nvidia driver hasn't been updated at least just doesn't clearly hold water. If you look at the right version number, it's pretty clearly moving ahead each OS release.
Apple would require Nvidia provide them the source to the driver so Apple could actually compile it themselves, so it makes sense that the Apple version numbers would be moving, and the Nvidia one, which would probably be set by Nvidia's compile process would not be. Apple isn't taking the built versions and just bundling them.
The other reason I don't tend to think Apple forked it is because Apple compiles the Nvidia drivers themselves, but they don't maintain them. Nvidia would have contributed the Metal driver. It would be also really weird if Nvidia actually wrote features like OpenGL 4.1 in a fork.
The built in drivers will lag behind the web drivers though just due to testing schedules, and Apple adding another testing process on top. Code freezes can happen weeks or months in advance of a release, and if Nvidia misses a code freeze, their changes won't get in.
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None of them have support for intel quicksync (FCPX fast encoding).
The low frequencies will make them slower than the iMac 5k in most creative apps. (Adobe CC, Logic ...)
These machines have built in GPUs. I don't see why QuickSync would be a requirement at all. That's why the Xeons don't ship with QuickSync. QuickSync is only needed if you're using integrated graphics, and those Xeons don't even support integrated graphics.