As far as size 8+ colonies, I'm not too worried about anything that happens past that point - I'd imagine at some point it will probably become exceedingly difficult to get to size 8, and impossible (or almost impossible) to get to size 9 or 10. That you can do so relatively easily right now is more of a rough edge of the implementation than anything else, so I wouldn't want to balance/fine-tune numbers for that.
Right now, the growth bonus from growth incentives is +15 per 100,000 credits spent. The amount of money you can spend doubles with each colony size increase... but so does the bonus it grants; this means that, past maybe size four, the actual base growth modifiers of the colony are effectively irrelevant.
Suggestion part one: Tone that down. Make the maximum incentive grant a +10, and make it so that the -cost- of getting that +10 doubles with each colony size, but larger colonies don't get a larger bonus. In addition, change how the slider works: instead of constantly ticking down, have the display show something like "100,000 credits invested, 20,000 credits per month to maintain this level of investment", and then it just stays at the level the player's set it at.
Suggestion part two: Add a scaling population growth penalty - call it 'emmigration' or somesuch - that's -6 per size above three. This would make each colony have a natural maximum size, and let the player boost a colony a few levels past that using growth incentives or the free port toggle. It'd also make decivilized more of a trade-off, with the scaling growth bonus from that trait significantly increasing the maximum size of the colony.
