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

Jesse, it looks like the examples you’re using are motors that are making more power with the 4412 to start with...... so a “better” combo to start with, where the 4412 is an even bigger restriction.
So, removing the restriction yields a bigger gain.

On the Busch North motors we used to service(358”, 18* heads, roller, Jesel, dry sump, etc), that had to run a 390cfm 4bbl........ swapping to a hp950 picked up almost 100hp.