I added the Threadripper 3 Maschine to my signature but its not possible at all to Hackintosh it, at least at the moment...
Thunderbolt will not run under MacOS, not only for a while... Not at all because of the integration into the bios!
Thanks to Gigabyte, they made the same garbage with the Gigabyte C621 Aorus Extreme aswell ...
If that wasn't bad enough, nobody at Gigabyte seems to want to correct that and you get the impression that it's wanted this way.
However, very important settings are missing, without which Thunderbolt will never run under MacOS.
Meanwhile almost 4 months past since I am in contact with Gigabyte...
The most incompetent company I ever had to deal with. Honestly!
So better hope and wait until Asus throws a decent board with Thunderbolt on the market or forget about Thunderbolt under macOS.
In Generel Thunderbolt Hot Plug and Thunderbolt function are in general a no brainer with a Titan Ridge on X99/X299/C621 Motherboards as long as a Thunderbolt AIC Header is present.
You just have to correctly apply the paths for the Titan RIDGE in the SSDT and everything will work like it should if it contains the important stuff
You can see it what is necessary aswell in my linked guide a couple of posts back in this thread.
If I can recommend C621 ? I can but its not that easy like many people will think about it.
I am still the only one running these machines, no matter if its with an Asus Dominus Extreme or the Gigabyte C621 Aorus Extreme.
If you need Thunderbolt try to get a Dominus Extreme, which will be hard to get, since there are none on the market any more.
In the past this motherboard had some issues...There were problems with two faulty resistors...
I had a total of 10 Dominus Extreme and helped ASUS to find the problem.
Only one mainboard out of 10 was not affected by this problem.
I found the problem and forwarded it to ASUS who took all boards off the market and fixed the whole thing (the resistors had to be soldered out and replaced).
Basically I can recommend you to use an X299.
There are of course differences in performance between the C422 and the C621 chipset, but these differences are not so great that they justify the high cost of a C621 chipset.
Apple use nothing special on the MacPro7,1 or the iMacPro1,1 chipset wise...
Apple does not make a special chipset, it is the same stuff as what is already on the market. Apple can also only cook with the water that we use.
The difference is in the CPU names, minimal variations here and there in the specifications, but nothing that matters.
An example for each of the builds you mentioned:
iMacPro1.1 - 18 Core Intel Xeon 2190B CPU is nothing else than the W-2195 CPU known in the server segment with identical performance based on the C422 chipset.
MacPro7.1 - 28 Core Intel Xeon W 3275M CPU is nothing else than the Intel Xeon W 3275 CPU known in the server segment with identical performance based on the C621 chipset.
If you would not need ECC I would recommend you the full way X299 since those are much cheaper and perform much better then an C422 Chip.
With a little overclock an 7980XE will out perform the new MacPro7,1 with an 28 Core without any issues at all.