I have discovered the joy that is the Superbreeder. I ran some test scenarios and looked at the population growth model and discovered why this ability is so powerful. If you keep taxes low in the beginning (because it's not much revenue anyway) or otherwise manage to keep morale at 100%, a new planet with 250M colonists will grow to 6.0B in ~17 turns, meaning you don't need transports to move population around, and your economy reaches maximum potential very quickly. Without the ability, it takes ~60 turns unless you do something to increase the growth rate or move people to the planet (which takes away from early-game expansion). With morale less than 100%, it takes 90-120 turns to reach the default population cap, so intervention is normally required to speed up the process. Since economy is usually the limiting factor in early growth, this ability really shines, especially if combined with +morale bonuses (which allows slightly increased taxes earlier) or +economy bonuses (better still). Researching Xeno Entertainment, Xeno Ethics, becoming Neutral, and building Harmony Crystals will yield +50% morale on all worlds, allowing a tax rate of 39% without really slowing initial colony growth. My usual strategy is to keep taxes at a level where native abilities keep morale at 100% until colonies are near the 6.0B mark.