Custom Video Card firmware has been around a long time. It was possible to convert PC video cards to Mac use by flashing them. When I was faffing around with PPC64 Macs (I mean hey I got it for free) I picked up an ATI card on ebay that had been flashed.
I had sleep issues. It was the lowest-end G5. I ended up recycling it.
Why don’t the OEMs of GPUs aimed at OCing just ship with a BIOS that exposes more tweaks through software though…