Assimilated populace gives a 15 % damage increase for 10 minutes to all allied ships in the gravity well of the newly conquered world.
That is very nice.
However, the increased population grow is absoluty worthless. It saves you arround 10 seconds of time compared to a planet without the upgrade has reached the maximum population level.
Suggestion: When this upgrade has been researched, give a new planet automatically the first population development stage and max out the population on this stage. That way it would make a difference.
So instead of the 10/10 population of a new conquered terran planet it would be 30/30 population.