I think alot of confusion is setting in here....... And one thing youns are forgetting is the load value or resistance value.
Agree, a 12V system can operate effeciently with a CONSTANT source voltage, not a variable voltage. I see more pilots go to a 12V battery onboard, with CC Pro BEC regulators, set to a constant voltage.
But, Im going to disagree with you because of the reason that if you keep your load constant, and you increase your DC voltage, your DC Amperage (current) does infact go up. Ex... 8.4Vdc constant source voltage dovide it by 12 ohms of load (resistance) gives you a current draw of .7A. Now, you increase your DC voltage to 12, load stays the say, your current is now 1A.