Yeah, I seriously question the utility of carrying a second mag. When you figure the average distance a typical shooting with a pocket pistol takes place at is well under 3 yards, I doubt if a reload during the altercation would even be possible. If you can still conceal the pistol with the +1 extention in place, so much the better. My pocket holster (handmade by some guy at a gunshow, no brand name) just covers the butt of the gun with the standard floorplate in place. The extension would likely be noticeable as my shorts usually are starting to curve around my leg at this point drawing the fabric over that edge of the holster. I don't wear tight pants or shorts but this still happens.
My feeling is that if I need a larger and more controllable (and less concealable) pistol, I would just bring a P11, Kahr PM9, or even a baby Glock rather than make my small pistol larger. Still the P3AT is so small that even with the extension it would still be a bit smaller than the pistols listed above, so I see your point. BTW, I have a PSP-25 (Browning Baby) in .25ACP I carry when even the P3AT is too big. ;D