This would deserve its own thread in the appropriate section, but the thread on W-3175X is not quit suitable either, so I'll reply here.
Beside PCI lanes, the benefit of "true" Xeons (not E3/E-2xxx/W-1xxx, which are regular Core CPU with ECC support) is
registered DIMM, which give access to larger capacities and cost less than UDIMM—
a lot less when buying refurbished or second-hand hardware from server upgrades.
ECC never hurts, but I suspect that stability benefits mainly come from server-grade engineering of the boards and a lot of tedious, invisible, optimisations of the platform.
As you appear to not seek extreme core counts, Xeon W-2xxx on C422 would be the natural candidate for CascadeLake-W. There is a long thread on another forum on C422 boards (mostly Asus boards, but also Supermicro X11SRA-F), including working Clover and OpenCore EFI folders. There are refurbished W-21xx Xeons (Skylake) on eBay for low prices, even including the custom Xeons for the iMacPro.
I found a fair price on a CascadeLake W-2255 and bought a
Fujitsu/Kontron D3598-B industrial motherboard, whose layout fits well with a modified MacPro case I had. Yesterday I dumped the ACPI tables, wrote custom SSDT for OpenCore and… watched Mojave installing without any intervention between the click on "Install" and selecting my time zone. More testing is required to check that it is stable and then try with BigSur, which will be the ultimate test for native NVRAM, but the first impression is very positive.
If you want to change for the thrill of changing, though, C422 may not feel exotic enough coming from X299. But then C621 is a very different beast.
Unless you need the 64 PCIe lanes, W-3xxx and the restriction to a few boards (Asus Pro WS C621-64L SAGE and /10G variant, Supermicro X11SPA-T) is an expensive proposition. (If base frequency is important, but 18 cores or less is enough, CascadeLake W-22xx Xeons are the rational choice.) I would rather suggest "regular" Xeon Scalable (Skylake-X/CascadeLake-X): There are more boards to chose from, and with a second-hand CPU you may not have to sell your X299. Which is just as good because C621 is a journey.
After my first
Xeon hacks on C246 (straightforward), I tried the Gigabyte C621-SU8 motherboard, with a Xeon Silver 4216 (about 500E second-hand, not bad for 16 cores). I took me months (not continuously!) to successfully hack it. Hopefully, I'll have enough time next month to make a complete write-up.
Short story: C621-SU8 works as hackintosh. Has native NVRAM. Sleeps fine.
Longer story: 180 ko of ACPI code in a single SSDT to
wrap the CPU in the form that OS X expects. A few more hurdles to eventually boot a pre-existing installation but not an OS X installer. Study ACPI code of the SSDT provided by Dortania. Study the DSDT. Find that SSDT-PMC applies => Get native NVRAM, and install BigSur. But still cannot install older OS Mojave and Catalina! Many unsuccessful tries. Frustration. And serendipitous success!
While updating OpenCore I turned off all booter quirks but the one I knew is required; this unlocked Mojave and Catalina.
C621, at least on this board, actually requires much less
less quirks than regular consumer platforms (or C246)—and I did not expected that.
I learned a lot hacking this board. If only for that, the journey was definitely for it.
If you want a computer to hack and learn, pick up a C621 board and go ahead.
If you want a computer to use, choose an easier platform—or wait for a complete write-up.
