I'll be honest, I don't even go for the Curia as a Catholic nation. I keep stability pegged at 3 and farm mercantilism. I actually think that the stability cost in PI being constant, while the stability cost in Adm gets very expensive at high end is OP for Catholics. It's particularly powerful for republics, who can keep president for life after president for life as long as they keep conquering and converting (to farm PI to buy stab with, because you sac stab to buy back RT). It feels a bit like a pyramid scheme, the moment you stop expanding you're buggered, but it is immensely powerful. Let vassals keep their own religion so that you have conversion fodder after you annex them. 100 mercantilism without paying any Diplo is very powerful as well, in a recent Italy run I ended up with a monthly income measured in "k".
I might suggest putting cooldowns on papal actions.