I'll experiment with the helper spring Joe. Where did you get/make the spring guide on top? It looks like it has a square top to match the underside of the mag follower. Nice.

Just to report in so far, I have had some success in leaving the anti-tip in and simply adding a couple of pieces of sticky back felt to the top of the follower in front. This causes the first round's nose to pop up a bit more. Naturally each round follows a bit further up. With this (humble) experiment, I think I can get 15 reliably. Taking it to the range tomorrow to try. I'll report back.