I just got my hand on a 3. MBP 15″ with the famous GPU issue, a late 2011 version, and I did some experiments. The first thing I did was mounting a brand new SSD and then:
1. Clean install of El Capitan. Everything works perfectly
2. Clean install of Sierra. Everything works perfectly
3. Clean install of High Sierra, F1/F2 does not work and sleep mode as well
In all 3 cases I have been using the latest release of the OS.
Safe boot did not change anything, but since it’s a clean install every time it makes sense.
I repeated this on my other 15″ MBP (early 2011) 17″ MBP and the result was exactly the same.
I have not been doing anything regarding KEXT’s ect.
Before the R8911 removal I found out that removing all AMD*.* KEXT’s and files was making the OS, no matter which version, act in the same way as High Sierra does with the R8911 removed.
With the R8911 in place i have removed the KEXT’s AMDRadeonX*.* and F1/F2 plus sleep mode works. This is why i think it is related to the KEXT’s in one way or another. I agree that it seems strange that the existence of some AMD related KEXT’s should have any influence on the issue. My experiment was just a long shot, but it worked 🙂
The big question is now: What’s the difference between Sierra and High Sierra?
I have attached 3 screen shots from the High Sierra installation and one from Sierra. 2 more will follow in next post