Since the cases roll on the mag walls and not ribs, like the 40 tubes, adding 2-3 rounds may just add enough weight and friction to to make a marginal setup fail. Even with tubes that were tuned by Grams were still "sensitive". I eventually converted to .40
It has been a long time but I seem to recall the Arredondo's pads offered one less round than the Grams. I think they worked though.
The guy who was making the mag catch was Canyon Creek but I have heard he has shut his doors. Any gunsmith should be able to weld some material on a metal mag catch. If I remember correctly, the factory catches were plastic.
This was my mag catch mod for the slim taper of the STI tubes:
And, of course, the Ed Brown part is out of production.
And, I just remembered this: Back when dinosaurs roamed the earth, STI made "for Para" tubes that put the notch were Para wanted. You may want to put out an APB for such things.