I have a weird issue. Downloaded the latest nvflash 5.287, tweaked my bios (extracted it first) and then flashed by doing exactly like OP. it matched the bios and card and asked to confirm to proceed, typed Y and it started, confirmed it was successfully flashed after that, re-enabled my GPU in device manager, rebooted. Extracted the BIOS with GPU-Z to check if it's correct and it's the same default one? So it actually didn't flash? I don't get it.
I have a Palit GTX970 jetstream btw