Most of the nations who are going to benefit from it are outside Europe, and therefore don't get cardinals. 5/conversion is completely fine in that case - they can't control the curia, but they can grab some stability or mercantilism or whatever. Otherwise Catholicism is awful for westernizing africans/americans/asians and CNs.
As it stands, catholic is THE worst Christian faith if you're outside Europe. It's pitiful compared to any of the others. The issue is that while the 5/conversion would mean a nice and reasonably balanced bump when these westernizers convert their home to the papacy, it would be brutally overpowered for someone still in Europe, with cardinals, who is plowing into foreign territory and converting in addition to their cardinals back home. Picture PLC, with all the cardinals they tend to accrue anyway in Warsaw, Danzig and Krakow, driving east and converting all those low-bt fields at 5/pop.