I have had no problems getting 15 into the mags on my .40 fullsize from the day it was new. Also, haven't had any problems with rattling. I have a ritual when loading a mag that involves smacking the floorplat of the mag once, and the back side of the mag a couple of times against the palm of my hand to nsure that the rounds are properly seated. I dunno if it helps but, it does make me feel like I've done all that I could to minimize problems.
For what it's worth... The mag rattle issue isn't limited to S&W mags. The most annoying mag rattle problem I've ever had was on one of my H&K USP .45's. It never caused any functional issues... Just irritated me. No such issues with my M&P.