Ok, but in Proportions, I expect this plan will take many years (maybe even decades or centuries) to pay off more than you could theoretically get if you could figure out a way to leach their trade income, because even with 100% added population for new colonies, you still have to transport the people around to the colonies using ships (maintenance), and to build the colony infrastructure to compete not only with 20% of your homeworld and empire, but with 20% from any other homeworlds they trade with.
Of course, I'm not at all sure how much of their trade resources you'd be able to leach from the AI, or how much your opponents would be able to leach from them.
PvK