I haven't tried this myself to verify this, but I did read it on HKPRO.com. During the ban, I did use .40 mags in my USPc 9mm and it fit and functioned 100% and I got 12+1. Word is that a 12-round no-ban .40 compact mag will fit and function with a 9mm compact and will give you 15+1 capacity.
For you 228 owners, I have tried this and it works: If you take the follower out of a 10 round 228 mag and install it in a no-ban 228 mag, it will increase the mag's capacity to 15 rounds. This I can verify from personal experience.
Anybody else know of any mag tricks?