Today I did some number-crunching regarding the colonisation of planets that Stars! calls "not habitable" and the player community mostly calls "red" [...]

A fully-loaded 100% world with its full 1650 factories built produces 2915 resources. To turn the world into a supplier for reds, as noted above, we drop it down to 50% hold (550,000), reducing the resources produced to 1458 (1457.5 rounds up) and sacrificing 1457. At 50% hold, the world will grow 16/9 * 1 * 0.19 * (1 - 0.5)^2 * 550,000 = 46,444 population per turn (call it 46,400). -15% reds at 100% capacity (55,000) kill 0.015 * 55,000 = 825 population per turn (call it 900). so our 100% world can keep a staggering 51 red worlds at that value. Each red builds 82 factories and produces 145 resources per turn, for a total of 7395 resources - 5.08 times the resources sacrificed! Obviously, this is an amazingly-good deal.