I am looking for the same answer regarding my mid-2010 iMac 27”. At this point I’ve successfully upgraded the CPU to an i7-870, added an SSD and lots of Ram, replaced a dead power supply, all using this great site. This machine is snappy and perfectly stable in High Sierra but to keep it going for a few more years (up to sync with all the latest services running in my more recent macs) I’d like to see if it’s possible to upgrade the GPU to a metal-compatible one.
There must be someone out there testing this hack…? Cheers, L