I think the reason they use 14 volt bulbs is because that's about what the charging system is putting out running down the road. I've only had to replace one in all these years and that was back when the bike was new. The 12 volts will work but probably burn brighter. And the lower the voltage the lower the watts. At 12 volts the 14 volt bulbs would consume about 1.4 watts.
Most sensors on the bike, like the TPS
, receive a 5 volt reference voltage. Whether that also pertains to the meter I don't know. What I have observed is that when the battery voltage drops to around 5 volts during starting the meter resets.