Population growth is currently calculated per time increment.
It logically goes down as life span increases, as an intelligent being is seemingly not more likely to produce more offspring if it lives for longer.
Thus, while total population goes up as more people live at the same time, effective growth rate would actually go down because it is the same absolute growth, being a percentage of a larger population, and stretched over a longer time frame.
In addition, looking at the negative growth rates in modern industrialized nations like japan, germany, etc, medical improvement and general prosperity seems to result in lowered growth rates in absence of measurable religious influences.
From a pure gameplay perspective, a lower growth is absolutely reasonable, for the aforementioned reasons, and because a longer childhood means it takes longer for new members of society to reproduce.
But if we go that far, we should do it for real, also including various traits that have effects on the population, to actually allow people to customize a bit.
Though, this opens the next can of worms, which I'm not sure should be priority.
An insect population, for example, would have extremely high populations, with high growth rates, and a lowered production and research, where the production is kept up by numbers. This would require them to have more workers per installation, and mean more deaths in the case of a nuclear exchange.