The G and B firmwares are the same only up to P9, part of the disc quality scanning code in the firmware was pulled from the G7T9 release, you will need to use G7P9 to use disc quality scanning if you do not want to crossflash to use any more recent firmwares & have disc scanning capabilities.
Officially, your warranty is invalidated if you install a custom firmware - but they probably won’t even bother to check.
DVD burners are so inexpensive now, that it doesn’t make sense to pay someone to check something like this. About the only thing they might do is to check the firmware version number - but I suspect they probably won’t even bother to do this.
If they can make a 6 Million $ lawsuit, of course they’ll be able to fine it.
But in most cases, flashing back to the original firmware completes the “perfect crime”. But moral issue still remains, as others said.
IMO, they won’t even check the firmware. They will just receive one and send the customer another one. I even heard of someone who found that his replacement drive was equipped with a hacked firmware.