More bullets is better but that grip length is the governing factor on cealmemt so really can you get away with 15? I don't think so tbh, but it's a great match mag or backup mag. Like others have indicated I think I'll stick to the shorter mags for carry- I always default to the lowest overall length magazines regardless of capacity hit with my ccw selections.
I think it's highly individualized based on body size and clothing (I just stated the obvious right?
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In my own experience, for being average height and build, carrying AIWB is the only way to conceal a grip of this length. With a dedicated appendix rig, I can conceal a G19 sized gun under most untucked shirts, no problem, because the long grip gets rotated in toward my body and hides itself along the belt line. At 1:00 I can carry my CZ P-07 without issue. At 4:00, the best I can manage is my Walther PPS with the flush 6-round mag.
While the CZ doesn't print up front, in the sense of creating the outline of a gun, it does create a lump. Now most people don't look "down there" and if they did would never suspect anything other than "that's a weird place to gain a few pounds," but the single-stack PPS creates a considerably smaller protrusion AIWB, and that's with the very extended 8 round magazine. It's amazing how much that 1/3 inch in width affects this. Under tighter fitting shirts, I drop down to the 6 rounder and any printing from the grip disappears. But I'd say for 90% of situations, I carry with a longer grip. If I'm wearing something like business attire that just screams for smaller, I drop down to a .380 and all clothing problems are solved.
Anyways, when I read about a P365 or (soon, supposedly, with aftermarket mags) Glock 43x packing 15 rounds, I hear that I can get the capacity of my P-07 in the profile of my PPS. So in my case, I suppose I very likely would carry with the 15 round P365 mag.
But yeah for 4:00 carry or some other position, I'd stick to the 10 round mags and use 15 for range or competition use.