I shoot a little every day. I have put well over 3,000 rounds thru my USPc 9mm in the past 6 moths or so. I own 1 9mm mag that came with gun, but don't use it. I use all 40cal 12 rounders, which hold 15 9mm. I only load 14 in mag and 1 in the tube, because with 15 in mag, it feels too tight in the gun. You have to press up on the bottom of the mag to help the mag release not work so hard when 15 in mag. With 14 in mag, it feels normal.
I have had ZERO malfuctions. But,,,I measured a couple 9mm mags, and the opening between the top lips is .006-.008 bigger on the 40 mags. I bent them in to match the 9mm, and have zero problems out of my 5 mags. Your 9mm might be tight for the 40cal,,,try it and see how it feeds. It will probably hold the 40 shell lower in the mag(slightly)