I'm really not sure if the mags for this thing will hold more than 10... seriously.
Yes, the follower is long, but have you looked at how the spring inside sits? It's just a little coiled up spring about the size of a "big" pen -- vs your skinny Bic pens that are the size of a pencil. It doesn't fully support the follower like you'd see on a standard magazine (well, every other magazine I own).
To make it work you'd not only need a longer spring but something that would support the front side of the follower, since you've "cut down" the follower. With a long follower like that the follower remains nice an stable even though it's only being pushed up from the backside. It'd take a bit of experimenting to figure out what kind of full-size spring you'd need to make this work. Not sure if Walther/S&W would even want to bother with it honestly.
Besides that -- start stacking up rounds next to the follower after you've removed it from the mag. How high is it? You can probably get 3 more rounds into the mag, maybe 4, if you cut that follower down to next to nothing.