Is slow expansion better in BNW?

With new cities now slowing your tech acquisition would you say that REXing and mass early conquest can be harmful to your empire? I'd also say that because there's less gold on the map new cities can rapidly become a drag on your economy too, unless you grow the population tall enough to benefit from city connections and to work your low food luxury tiles, instead of just delicious river tiles.

Can you grow wide so rapidly that all those burdensome cities actually drag the empire under? Does this make uncontrollable puppets a hazard?

I'm thinking it would be best to get your house in order every time you get a new city, instead of just rapidly gobbling up land. Grow your capital, settle/conquer a city, grow it and build it's infrastructure one city at a time. Slow, steady expansion.

Is this why a lot of civs have had their expansionist flavours toned down?

Is there a good rule of thumb as to how much science a city needs to produce to break even? Does each city have to reach and maintain 5% of the capital's output to not be a burden?