Got back from the range a little bit ago. Put 400rds downrange on the PSD PDW, and about 70 downrange on the 18" SPR. The PDW is still running strong. Not a single hint of malfunction after 700rds total. I DID put in an H3 buffer, and that seems to have helped with slowing the bolt down even more to reduce damage to the case rims.
The TangoDown ARC mags work very well, as expected. However, I really dislike how frickin' bulky they are. Two cannot fit in a double-mag pouch, and that's annoying. I would have to say that given the issue of just how wide they are, they're just not feasible for a combat load. I'm still waiting on my mag shingles for my new chest rig, so we'll see how they do there. Maybe problem solved?