It's not a gimmick when the serious overclocking people do custom VBIOSes. Similarly for miners. That's when you need the recovery switch if things go wrong.
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.
I was under the impression that GPU overclocking, overvoltage and fan curve settings were not persistent across reboots.