Well if the goal was to slow down the initial colony rush I don't think it worked. Everything I have seen that is colonizable has been taken before turn 50. If I didn't take the ones near me, someone else already had colony ships on the way (including Mars in the Sol system).
So the planets are all taken, but with minimal pop on each. Research or production doesn't seem to have suffered as a result (as I said, I'm getting techs almost too quickly and I can building any ship in my tech base within about 2-3 turns).
And honestly a massive hit to pop growth doesn't seem to help Tall at all. It will take forever for a planet to grow to a decent pop level and since you get at least minimal output from the colony bases it still behooves you to grab as much as you can, as quickly as you can.
A few other things of note:
'Wealthy' is extremely beneficial now. +1 production on every colony when most colonies are making 1-3 raw is a HUGE increase.
Asteroid Bases are far more important than anything else now. My homeworld makes 8.57 production, FIVE of which comes from the nearby asteroids. That means that asteroids are basically make or break...if you have them, you're on easy street. If you don't you're likely to fall way behind. And what's the best way to get Asteroids? Yep, you got it, take every planet you can that has them nearby.
So yeah, I still don't see the benefit/rationale for reducing population growth buy such a severe fact. Maybe .05 instead?
As it stands now, population has gone from a major contributing factor to the economy to one of the weakest. I'd much rather than 5 colonies each with 1 pop and an Asteroid or two than 2 planets with 3 pop each and an asteroid or two. The former will produce 15-20 raw and the latter 8-10 raw. It's just no contest and it seems even more tilted in favor or wide now.
If you want to slow colony rush, remove the huge wad of cash that everyone starts with and make colony ships cost significantly more. That's the way MOO2 did it and it worked. You simply weren't able to produce large number of colony ships until you had tech'ed up and developed a bit. They were a massive drain on econ. Of course in MOO, you had to make the choice between production or science which is not necessary anymore in GC3 but even still if a colony ship took 15-20 (or more) turns to build you'd be a little more selective about where to send them.
Alternatively, you could make population growth a fraction of available pop on the planet. So then keeping a decent pop base on the world might pay off more short-term profit than flinging them out in penny-packets to every available world where it would take a long time to realize any benefit.
But as it stands in Retribution, the changes seemed to have ENCOURAGED colony spam and Wide, not discouraged it at all.