I've played a couple of games now and one thing that I'm still unsure about is just how useful it is to specialize planets into primarily producing one type of resource.

Basically, it seems to me that, considering almost all improvements provide a percentage increase that only stacks additively, and that the base production of a planet is based primarily off that planet's population, when it comes down to it, you want to place your high % unique improvements on whatever planet has the most population.  Is that true?

For example.  Planet A has 20 population but is pretty much covered in manufacturing buildings.  Planet B has only 5 population but is covered in research buildings.  When deciding where to place a unique improvement that adds 200% to research, putting it on planet A will make that 200% bonus add more total research to your empire than placing it on planet B. 

Now, this is counteracted somewhat by the adjacency bonus system.  If Planet B has good enough tiles to have the 200% building completely surrounded by research centers, then this changes things. But does it change things by enough?

Let's do some maths based off of numbers that are not necessarily true (if anyone knows a place that lists all buildings and their effects that would be a very handy resource).  The planet 20 pop, let's say, produces 20 base research.  So putting the building there means an increase of 40 research per turn for the empire (20 * 200%).  The 5 pop planet produces 5 base research, but has a 6-way adjacency bonus from placing the unique.  Let's say the unique adds 2 levels to adjacent research buildings, and I think as a rule each level grants +5% reasearch.  So that's +10% for each of the 6 research improvements, for +60%.  Going the other way, each of the 6 research buildings adds a level to the central unique, giving it 6 extra levels for a +30% bonus.  So the total research bonus being added to the planet by this unique is 200% + 60% + 30% = 290%.  So at the end of the day, placing it here, in this perfectly specialized but lower populated planet grants you an additional 5 * 290% = 14.5 research, only a third of what you'd get from building it on the completely unspecialized planet.

So, based on this, even a perfect tile-specialization setup won't make a low-pop planet outperform a high-pop one.  Planet B would need at least 13.7 population to break even with planet A.  At equal population, though, planet B gives 58% more research to the empire if you build the unique there compared to A.

In other words, if these numbers are even vaguely accurate, you should be very careful where you build your unique buildings.  Just because one planet is littered with a specific improvement type doesn't mean that it's the place to put your unique, 1-per-player building. 

What does this mean for planning?  Well, to me, it feels like you should never really specialize a planet completely in this game.  Instead, you want every planet to have as high a population as possible, prioritizing growth and food cap, and then have multiple clusters of different improvement types based on tile bonuses and the layout of the tiles a planet has.  Choosing the right planet for unique buildings under this scheme seems like it takes a lot of calculation, but if you're focusing on getting as much population as possible on all your planets, then the choice should be made to put these improvements on whatever high-pop planet has a good enough layout to provide plenty of adjacency - even if that planet is otherwise not particularly specialized.

Hopefully that doesn't read as too much of a wall of text - kind of explored the idea as I went along there.  Would love to hear some thoughts from more experienced players on this, if anyone has any.