I've seen A LOT of posts about making HP with a 318-So why is it so ???

Looking at it from the other side...... it’s not how much you gain from the 4bbl....... it’s how much you loose from the 4412.

On a stock low CR roller cammed 318, the 4412 might only cost you 15hp from the factory TQ.
At 5k the stock motor probably isn’t even using 400cfm, so the 375cfm @1.5” carb isn’t much of a restriction.
(Edit- I looked at the sheet from my 30 year old test...... while making 293hp@5000rpm, with a 4777 carb..... the motor was only using 417cfm. So the stock 318 would be using even less.)

On a 900hp 572 that runs a 1250 carb....... and uses most of all of that........ it’ll probably cost you 200hp.....maybe more(probably noticeably more).