Right now, from my understanding, there are cutoff points for both when citizens begin leaving your empire due to low popularity as well as when you get a population-growth bonus. I think it would be more realistic and much MUCH more interesting if there was gradation of population growth and people entering/leaving your empire. In other words, if those two things were variable based upon your popularity. At popularity 65%, for instance, there would be slightly increased population growth or at popularity 40% a small number of people would begin to leave, for instance. This wouldn't have to be a linear progression, it would probably be logarithmic or even an exponential progression, i.e. a small growth bonus at 70% and a much larger one at 80%.

This would provide a strong incentive to keep your popularity level up, especially if you try and tax the hell out of people. It would make things much more realistic and MUCH more interesting as opposed to avoiding the cutoff of around 30% or trying for the 100% mark on planets.