I have been 100% pleased with my LCP and other than the normal ramp polish that I do with all my semi-autos, I did nothing more than fire it to break it in. As for the magazine extensions, you should stay away from the Ruger provided extensions as they put undue stress on the magazine during recoil, which can cause the next round not to chamber. The only mag extensions that do not seem to cause this condition are the Pearce Grip extensions. You can find them here for $12.95 per 2 pack. Elsie Pea Store - Accessories for the Ruger LCP
I have fed just about every type of ammo through mine without any issues. I have settle on American Eagle 95gr for my practice ammo, and Remington Golden Saber 102gr for my SD ammo. Why these two choices? The American Eagle because it has been shown to consistently penetrate deeper than most other .380 FMJ ammo... If I ever need to use it for self defense. The Golden Sabers because they been shown in testing to have the weight, expansion and penetration attributes closest to 9mm Luger rounds.
Have fun with your LCP. I'm sure you will love it like I love mine.