Okay, I installed the update, set the "RefsEnableLargeWorkingSetTrim" registry key, rebooted, and after some time got an avalanche of errors that seemingly started with:
"Bus reset occured on storport adapter (Port Number: 2)" (from Source:StorPort)
Following that, a bunch of disks dropped out, Storage Spaces unmounted itself, and then a cascade of further errors kicked up. I rebooted the server, and everything looks normal again. This may have been a wild coincidence, but....doubtful. I'll wait and see if it reoccurs, but...yeah. Be careful, folks. For sure, don't set this on your primary servers until there's more feedback.
Incidentally, I'm using a custom view in event viewer that might be helpful. It's an easy view of everything refs/storagespaces/disk/etc related, only a small fraction of which shows up by default in the normal syslogs:
Event Viewer -> Custom Views -> Right Click -> Create Custom View...
Event Level: Critical,Error,Warning
Event logs: Microsoft-Windows-DataIntegrityScan/Admin,Microsoft-Windows-DataIntegrityScan/CrashRecovery,Microsoft-Windows-Storage-Disk/Admin,Microsoft-Windows-Storage-Disk/Operational,Microsoft-Windows-Ntfs/Operational,Microsoft-Windows-Ntfs/WHC,ReFS/Operational,Microsoft-Windows-StorageManagement/Operational,Microsoft-Windows-StorageSpaces-Driver/Diagnostic,Microsoft-Windows-StorageSpaces-Driver/Operational,Microsoft-Windows-StorageSpaces-ManagementAgent/WHC,Microsoft-Windows-StorageSpaces-SpaceManager/Diagnostic,Microsoft-Windows-Storage-ClassPnP/Admin,Microsoft-Windows-Storage-ClassPnP/Operational,Microsoft-Windows-Storage-Storport/Admin,Microsoft-Windows-Storage-Storport/Operational