But do they work much better by providing bonuses to your planets in the area of effect? |
Define "Work much better." If the point is to flip a planet, then it would only work better if your planet was near to theirs. An influence starbase doesn't actually change the influence a planet generates; it changes the influence of the parsecs in its radius of effect.
Yes, if you happen to have a nearby planet, it means you need fewer influence starbases to get the effect you want. But that's because it's magnifying the influence that your planet is already casting, not because it's changing the planet's influence generation. Multiple influence starbase effects do accumulate very quickly.