After some Googling, the error appears to be in the permissions of the .kext files and they need to be "rebuilt". Unfortunately further Googling has lead me to the conclusion that the Terminal commands used to repair kexts in the past have been removed from Sierra...
I'm at a loss for what to try next. I suppose I might be able to get the three 1.4.8 kexts to load by simply adding them to the Extensions folder as you two have done, but then there's no way to load the X3000 kext after I've logged in.
I guess I can only tell you what I did...
Originally I had moved the AMD (1.51)*.kexts to a folder called AMD_kexts, and did the EFI thing...
sudo nvram fa4ce28d-b62f-4c99-9cc3-6815686e30f9:gpu-power-prefs=%01%00%00%00
After learning that there are three Kexts that have to be replaced with earlier versions:
I pulled them off an old Sierra install USB... Which gave me 1.44 versions After disabling SIP, I booted into Recovery, and copied them to the /System/Library/Extensions... With the following commands given to me by nsgr:
Copy AMD kexts (1.44) from pendrive to hard disk - use TAB key to autocomplete name directory/folder:
cp -v -r /Volumes/OS\ X\ Base\ System/System/Library/Extensions/AMDSupport.kext /Volumes/Macintosh\ HD/System/Library/Extensions/
cp -v -r /Volumes/OS\ X\ Base\ System/System/Library/Extensions/AMDFramebuffer.kext /Volumes/Macintosh\ HD/System/Library/Extensions/
cp -v -r /Volumes/OS\ X\ Base\ System/System/Library/Extensions/AMD6000Controller.kext /Volumes/Macintosh\ HD/System/Library/Extensions/
On reboot, Selecting Apple/About this Mac/System Report/Software/Extensions showed the 1.44 versions were now recognized and loaded.
Now the next step was to load the X3000 (v1.51) kext...
No need for single or recovery boot here... Just run Terminal as usual...
After login, load AMDRadeonX3000.kext - in Terminal:
Remember, AMDRadeonX3000.kext moved to /DisableExtensions directory.
cd /DisableExtensions
sudo kextload AMDRadeonX3000.kext
Note: These were nsgr's instructions. My disabled AMD (1.51) Kexts were in a folder called AMD_Kexts, so the above two lines where changed to
cd /AMD_kexts
sudo kextload AMDRadeonX3000.kext
As soon as I did this, temperatures plummeted...