Quick update on ME firmware. Disable AMT in the BIOS or by the header and you can ignore the ME firmware version. Just upgrade your bios to an appropriate version (eg v3.85) and flash your boot block with the 2013 version.
I got my v2 test CPU (an astoundingly slow E5 2603 v2, 4 cores @ 1.8GHz. It’s about as fast as a dual core laptop i5) working with 2013 boot block and AMT disabled. Worked for getting into BIOS, using FreeDOS, using linux desktop and server.
I did also upgrade to ME v8 firmware by replacing the ME region in a dump of my BIOS with the ME v8 region of the v3.85 BIOS update file. It shows up correctly in the BIOS. I didn’t get it working with the v2 CPU, but I think that was down to trying to initialize the upgraded ME with the v2 CPU, when I should have been using the v1 CPU then swapping in the v2 CPU after.
So, for all considering a Z420/Z620/Z820 v1 for the purposes of upgrading it to use v2 CPUs (all…15? of you), and worried about needing to upgrade the ME firmware - you needn’t bother. Enjoy the wonderful AMT-disabling motherboard header HP has provided.