Call me cynical, but I doubt the benefits of any such dramatic breakthrough would last long. As soon as the battery ability expands, the hardware and software will begin to expand their battery demands as well.
Maybe in software, but I doubt that the efficiency of motors in drones or cars would change. It's not like there are abstractions to chase in hardware, are there?
It will decrease. Making 99% efficient engine is order of magnitude more expensive than 90%. Double the battery density and suddenly the case for lighter and expensive parts disappears.
That wouldn't be decreasing, just increasing at a slower rate.