Energy consumption and cargo size are completely irrelevant on weapons that are used in turrets only, turrets have neither.
It would make really interesting gameplay for large ships to have either global or per-turret energy reserve and ammo capacity. It would add another layer to tactics.
We're using theoretical maxims about power vs energy consumption vs cost to justify existing balance decisions - it's all just mild technobabble to give the turret gun system a little context. If the game processed actual ship energy values, the only one who would have to deal with it would be the fredder - and anything he/she does to the player in response to said energy system would sound like another gameplay conceit. Thus, any tactical depth would still be dependent on a fredder's ability to explain, in-game, why something is happening (Oh noes, our reactor control rods are fracturing under the stress yada yada).
Now, if There had been a lot of additional 'technical' information about capships and their weaponry from the very beginning (and it made any sense, as opposed to just being flavor text), then it might make sense to have the game keep track of such things, so as to provide more dynamic combat between caps, without a fredder's guiding hand. As it is, any interesting battles between fleets (fleets here meaning the FS1/2 style 2.5 ships-per-side and some fighters) always involve careful planning and creativity from the mission designer - a fact that would not change if the game were capable of monitoring some sort of power-consumption value.
Having said all that, I can see having ships that are all about the first volley, and then retreating, in an alt-universe context. But at the very least, it's the sort of idea that asks a developer to handle a variable that the fredder can already handle in a meta kind of way, and the player would never know the difference.
It would be a very different sort of game that makes use of a more complex capship platform, more like FS2 -> Windmills than FS2-> War in Heaven .... planning anything interesting, fury?
because then that idea would be a precursor to something awesome. I'm all for that.