Wave, aside from the springs being weak, did you compare the followers side by side? I'd bet if there were a little flash on the plastic from the molding, a little sand paper would take care of it.
I just got a ProMag 20 rd 9mm magazine, and it looks alright. I has a plastic shroud on the bottom like the Beretta 20rd magazines come with. The back side (back of the magazine) is not shaped/angled at the top, so it hits the magazine adapter in the Storm. A little file work will remedy that. If I take the shroud dealy off, the magazine seats fine in the gun, and locks the bolt back normally.
As for how it feeds? I don't know yet. I might get to the range tomorrow afternoon or evening.
I also got a couple MecGar magazines. They look pretty nice, and have a much better reputation than ProMag.