Why is planet specialization inefficient? I agree that some planets can be general-use without hurting anything, and it may even offer flexibility. However, since the Capital improvements are a flat 100% bonus, you will get more bonus, the more capacity you have on a single planet. I try to make my tech, productiion, and economic capitals from large, specialized planets with as many of the same type of building as possible. I get research capitals that produce a third to a half of my entire research capacity in the early stages of the game. Even if not all that capacity is used at any one time, it's still 33% - 50% of my entire research production at that point in the game.
As for the non-capital planets, I don't worry about specializing them, except to have a few secondary production planets to provide extra ships.
Thanks for the great rundown, PirateRob. Would you care to throw the individual planet production into the mix and give us examples of the impact of those? I'm referring to the focus buttons on the planet screen to switch a single planet between emphasizing research, production, etc.