The book German Destroyers of WW2 by Koop and Schmolke says on page 35 that the 150mm mount had manual train and elevation, and they give the weight (450kg) of its "electric drive", from which I infer that it had electric motors for train and/or elevation in normal ops.
Regarding the rule: I would guess that it takes into consideration: (1) Fixed vs. two piece rounds, (2) weight of shell and powder, (3) powered vs. manual ramming, (4) powered vs. manual train & elevation. The gunnery hit rules assume a certain rate of fire, and perhaps the ability to lay the gun using a director. Without power, (of whatever type), the rate of fire is reduced, and director control is impossible. So rather than have a separate local-control (manual) hit table, the larger size guns are considered combat ineffective for game purposes if they are not powered.
Regarding your examples, I would say that pedestal mounted submarine guns, which had no power rammer, did not need power to be effective per the rules. Also, perhaps any pedestal gun. Any other gun, esp. those with a gun house or turret, are subject to the rule.
From Wikipedia articles on USN guns--
5in/38: Shell weight is 55lbs. and powder is in a metal "cartridge" (semi-fixed)
6in/53: Shell weight is 105lbs. and powder is in a silk bag weighing 44 pounds. (bagged)
