While we can run the MP5,1 firmware like this we do have a hybrid rom in the end that seems to confuse the macOS installers. At this point since the hardware is not officially able to run 10.15 / 11 and we have to use patched installers I don't think it is a big deal.
Sounds like you either go this route and keep some of the xserve's identity or you flash the blank 5,1 ROM and have other issues.
For me, this seems like the safer play (hybrid) if you really just want to get better CPU support and can work around the behaviors.
Note PRAM/NVRAM reset now uses the same thing the mac pro does (CMD+Opt+P+R) not the front panel option.
NOTE: macOS 10.14.6 will not boot even on the 144 firmware using this flashing method, it gives the "no entry" sign.
This means we aren't fully seen as a 5,1 somewhere (maybe board-id?) so it will refuse to boot without the nvram entry for "-no_compat_check" and it still panics during boot of an existing Mojave install that worked fine on the xserve firmware, but it was able to boot to a 10.14.x dosdude installer
EDIT to avoid panic booting 10.14 remove DIMMs from slots A4 and A5.
funny side note ... it does boot to 10.5.8 just fine (probably due to -no_compat_check in the nvram), Leopard boots so fast from the SSD!