Theres an aftermarket mag made by Promag, its a 15rd piece of crap. I've heard from tons of people who say it worthless and isnt reliable for anything. Personally I dont think 5 extra rounds is worth the hassle.
You have failed one or more fundamentals of proper shooting if you fail to put 8 (or 9 if you have one in the pipe) rounds into center of mass. Stick with what works, and keep it simple. It is better to have 8 rounds that feeds 100% of the time, versus 15 rounds that feed 70% of the time. If you are looking for a pistol that has a 15 round capacity, I suggest your research the Ruger P-series pistols, or something versatile like a Glock 19, Springfield Armory XD, Smith and Wesson M&P, etc....
The worst thing you can do is to try and "fix" something that isn't broken.
For the ten years of active duty I served in various parts of the world, I never carried more than 8 in my sidearm, less weight, hence better acquisition of target, and I didn't often miss, large cap mags are great for target shooting if you like to put lead downrange, but they are not a plus to actually neutralizing a threat, I always found them to be out of balance for my style of shooting, causing more misses than hits..
Give 8 in the mag and possibly one in the pipe and I will give you 8 or 9 downrange that hits the bad guy..on that note, my wife who has put down two bad guys in the last 15 years, only used three rounds to do it, the first one she hit his metal cig case over the heart and had to give him another one, the second one didn't smoke, so guess what!
26 years service, 10 active, 16 years training others to kill the bad guys