They also can use the S&W 59 series mags, and you can buy a plastic spacer for the 59 or 69 series mags from Kel-Tec that fills the gap and extends the grip. The 59 series mags are 15-rounders, and pretty common and relatively inexpensive.
The P-11 is a very small gun. The 15 round mag only adds about an 1 1/2" to its height, and with it installed, it is still far more concealable than nearly any other 15-round gun. Not to mention lighter.
I guess we'll have to agree to disagree on this point. IMO, the grip size it the most important part of concealment issues, and making the grip longer doesn't make much sense in this gun. In addition, if you haven't solved your problem with 11 rounds, you should have brought more gun to the gunfight.