First off, BIG thanks @MaLd0n! He stuck with this goofy setup and helped me a lot.
I tracked down some E5-2667v2s from eBay and dropped them in replacing the E5-2690s I had. Immediately my Mojave install broke, but using the iMac12,3-based EFI folder that MaLd0n created for me Catalina suddenly works great. The only niggly bits I have not fixed so far are:
Hyperthreading doesn't seem to work.
The i350 NIC had all sorts of issues with hnak's AppleIGB.kext. Any thoughts on flashing the eeprom, etc. to change the subsystem ID to match the Small Tree drivers? Any other good solutions?
The LSI MegaRAID set still shows as an external device. I've not figured out how to fix that.
Net/net I highly recommend ditching Sandy Bridge CPUs.