Rather than go into trying to fix your problem, which at first glance sounds like a incorrectly made, or wrong slide stop (9mm slide stops have a deeper lug to engage the magazine follower and you say you see no difference... so Taurus may have installed one for a .45)...
This seems like the easiest/best place to start. Remove the barrel/slide from the gun, reinstall the slide stop. Lock a mag in place. If the mag follower slips past the slide stop (9mm mag, original slide stop), then it does sound like there's a problem with the slide stop itself.
If the slide stop pin hole were even slightly mislocated, I'd expect to find all sort of other problems, as virtually every frame dimension is based on its location relative to that hole.