Would it be possible to change the population growth rate from the current system (I think it's like the minimum of 20/P^(1/3) and 0.1 up to a third of the population capacity, at which point it is a linear fall-off) to a logistic equation? Wouldn't be much more complicated than the current method. Would be simply r*P*(1-P/K), where P is the current population, r is a growth rate constant (that variable we put in per species, fudged a bit to make population grow sufficiently fast for a 4x game), and K is the population capacity (calculated the same as currently). That way instead of current (where colonies have crazy-fast growth rate at first until leveling off), it would be a nice logistic curve: slow to start, fast in the middle, and then smoothing out at the end. Plus this accounts for if the population exceeds the population capacity; at that point the equation becomes negative.
Actually... in the same suggestion, would it be possible to replace population capacity in that equation with min(population capacity of planet, population supported by infrastructure)? That way colonies don't outpace the infrastructure being put in place, and actually account for it in their growth.
Computationally-speaking, I think this should actually be cheaper to calculate given that it avoids that cubic root.
This should also play nicely with the "colony cost range" effect of the new eccentric orbits, given that the population supported by infrastructure would vary as well, and that this equation can handle going negative just fine.
If you want a good estimate for the current actual human population growth rate, assuming that the equation is working with millions of population as a unit, r should be about 0.00037. That should give roughly a population growth rate of 1 for 7.9 billion people where the population capacity is 12 billion: where we are now.