Okay, so I just got my first 20-rounder from Tapco and I gotta say I was a bit disappointed. I had my sandpaper and wood chisels all out and ready to go, but the darn thing just snapped right in there snug as a bug, no alterations needed. It feeds perfectly, and I even loaded 10 rds into it with a stripper clip without trouble (I decided not to press my luck trying to load in a second one). My only complaint was that I didn't quite realize how big the 20 rd magazine was (one of the reasons I'm not all that interested in a 30-rounder is because of how ugly it makes the gun look); I thought maybe the gun would still retain some of its elegance with a medium-sized mag on it but nah, no such luck. Just a minor cosmetic quibble. That, and apparently you have to empty the whole thing before you can remove it and switch out mags.
According to Tapco, their 20-round magazine counts for 3 U.S. made parts, so all I'd have to do now in order to have a legal firearm is find myself a U.S.-made gas piston. That, or I could spend a little more and get that spring-loaded firing pin, which I actually planned on getting eventually, anyway.