@MaaseyRacer you'll be pleased to know that after much head scratching and thought, I am currently installing Sierra. It wasn't the GPU (although the 1050 I borrowed is installed now) but I reflashed the EFI (same version 3.30) and that seems to have done the trick.
EDIT: Almost there!
Whats working: GPU, USB3, Sleep (but it kicks the USB so it's disabled right now), Ethernet (only one of them), Speedstep and CPU PM
What's not: Useful sleep (as mentioned above, it kicks USB and safe sleep USB causes a reboot), Audio (shown in sys prefs but doesn't seem to output), USB2
I probably won't use sleep, but it would be nice to get it working. Same with USB2. I might ditch internal audio and use a Griffin iMic I have from yesteryear.