Since the 5,1 firmware isn't getting any more updates, should we be looking at the MP6,1 ROM for updated APFS, NVMe EFI components?
It's seems we are back to hacking the ROM or sitting on the sidelines. Personally. I'd hate to see the 5,1 start gathering dust almost as much as you and many others on MacRumors.
Up to now, nothing in the BootROM improvements/modifications Apple has sending make 144.0.0.0.0 incapable of running Catalina DP9 or have any performance issues that I know of.
I have been tracking BootROM for all non BridgeOS Macs for more than a year and MP6,1 BootROM improvements have been very slow, it got just
one update after the equivalent of 144.0.0.0.0. I'm paying a lot more attention to others 2012 Macs with IvyBridge processors getting frequent updates, like MM6,1 or some of the iMacs (IM13,1 and IM13,2).
In the future, some of the APFS EFI modules like APFSJumpStart or the NVMe EFI support maybe require that we return to injection and but for now it's definitively not needed.
NVMe injection era was very problematic since most people are incapable of even grasp what is needed to do it correctly and if we ever need to return to it, will be needed a very different approach and with a different set of tools, not the current ones that double inject and make a mess.