There goes a saying between me and a friend: "Catholicism OP".
I just did a fun match against the expert AI on Amazonia. The goal was to have at around 30:00 no villager population and a working economy. It worked indeed. Some religions give way too much gather rate, giving an unfair disadvantage to civs with no such religions with enormous gather bonusses. Having huge gather rate also means a lower villager count is needed, and thus more warriors. And in the case of Paraguay, that means that they can run an economy without works. I had a military unit high of 203 (3 ships included) without going over the pop limit and without any natives etc; just regular military units. It was fun to swarm the enemy with that many units and see them callopse (game ended at 48). I had a strong economy and the gigantic gather bonusses allowed me to have a full koygua (87) economy. I admit, the conditions were made ideal, but it seems doable to do, since the starting gameplay was almost as usual.
I think a number of 50 to 75% should be the max total amount that religion gives. (something as or slightly stronger than imperial techs for gather rate).
What do you think?