I would actually support giving them that card instead of food trickle, because off 50 vills it's worth only about 5 more vills
That doesn't work as you are saying because economic cards never influence the modified values. That means your economic theory only boost their eco by 3 or 4 percent when you also use other economic cards.
So the effect of this card and the trickle don't differ that much.
Yes it does, the longer the game runs the more the value of the 10% card because even though they don't stack perfectly, they stack more than the trickle. Every single economy card you send makes trickle worth less and less villagers, in Imperial the coin trickle with Spain isn't even worth a single vill, the same goes for the food trickle with Dutch.
Economic theory builds and builds, the more villagers you have, the more it's worth, the more economic income you generate, it never looses effectiveness, a trickle card does.