I don’t think it’s right to assume that a conservative Catholic family will have 9+ kids.
Yes, generally, Catholic and Mormon families have lots of kids. However, the number of kids is more reliant on social trends than religious affiliation. Using NFP, which is just as effective as most ABCs, most conservative Catholic families have 2-5 kids, with most of them having 2-4 kids.
If every woman within her childbearing years in the world had that many kids, the population would continue to rise for a few generations due to demographic momentum, then…we don’t know.
Most peripheral countries today are in stage 2 of the DTM (demographic transition model), where mortality rates fall due to better technology but birth rates remain high from the first stage. Lowering their birth rates would take them into stage three (demographically), though the development of technology wouldn’t match. Which doesn’t make sense.
For a core country like Japan (hypothetically in stage 5 of the DTM) to increase it’s number of children to around three would take it backwards into the DTM, which doesn’t even make sense because it’s technological development wouldn’t match its population growth.
Anyway, I’ll stop there because I don’t want my brain to explode. You could explore the issue from many different standpoints; I chose the DTM. I know i didn’t exactly answer your question, but I didn’t want to go looking for percentages of the world in different zones of the core-periphery model, trends, or do a ton of math stuff that would make up a complete answer. Sorry

I would, because now I’m curious, but I’ve got work to do.
Your question extends far beyond a simple calculation and a couple assumptions on the population of the world and mortality rates.