I'll stress again that you don't have to take a planet to stick a mkII energy plant on it. For up to diff 8 at least, its possible to stick a small number(<30) turrets(mix of laser/mirv/short range I's) and a mkII power plant on a mk I or II hostile planet that you never plan on taking against most ai types. With only about 4-6 planets, you can have the same number of full efficiency mk II plants as someone with 12 planets.
Having more than 1 spacedock is generally a good idea imo, but most of them can be low power most of the time(and only take 1k energy then) and only turned on for emergencies. Early on I find I have more problems getting either crystal or metal for my space docks anyhow
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Regarding the 8 home planet case, that's pretty clearly not how the game was balanced for. If it was a real 8 player game, each of those 8 starting planets could have a mk II energy plant for each player at full efficiency.
Edit: On a more related note, they seem to be fixed(at least for the player). Not sure if its almost too much of a nerf(only time will tell there), but they are pretty useless at the start, as your economy cannot support them in the numbers required, and they die pretty fast. I'm getting to the point in the game where my economy can support them. Trying to figure out if what they save me in time/other losses makes up for their cost or if I would just be better off sinking those resources into mercs, but at least they are semi-viable at this point for raids. Part of what has devalued them in this game is that I was given armoreds as my first unlock, which means that I don't need them for turret killing nearly as much.