I was perusing the entitiy files and happened upon the reason the population gain buff on assimilated Populace is so underwhelming- it doesn't work....at present it does nothing.

To elaborate at present the assimilated populace buff entity file doesn't even try to buff population generation rate- it tries to instantly add 5000 population to the planet 0 seconda after colonization(at which point the planet is obviously still capped at 10 population).

I tested if my reading of the entity file was correct by adjusting the delay to 300 and lo and behold upon testing in game, 5 minutes after colonizing a terran planet(which I used the first few minutes of ownership to increase the population cap to 280) after 300s the terran planet's population jump from roughly 70 straight to 280.

So at present assimilated populace is currently trying to add 5000 population to newly colonized planets  at the time of colonization rather then giving an population generation rate buff.

This also means that the buff to Assimilated Populace in beta patch 0.80.0 didn't actually make a difference, as it adjusted the population bonus from 1200 too 500(both of which are enough to pop cap any planet, which doesn't matter anyway as they are applied to a newly colonized planet with a pop cap of 10).