When you do a firmware update on an RNS part of the process is updating the way it interacts with the can gateway on firmware newer than 1200 this stops the gateway from powering down the RNS so it stays running in sleep mode draining your battery.
There are edited firmware versions by Josi off of VW-Navi that he has cooked so that 1200 can firmware is parcelled inside the newer firmware releases, allowing you to use newer firmware with an older gateway.
I have now used 1300 cooked firmware and just recently a newer one that allows voice control, both without any battery drain issues and I have an F gateway. So it can be done, one other issue with updating the firmware on a A unit is the inability for it to store your last GPS location, hence why it always defaults to germany, there are firmwares which allow it to update quicker but still for the first 5 minutes the nav wont know where you are.