changing anything from factory on a ccw piece is I'll advised, if you ever did have to use it that can and will be used against you if you go to court. I'd recommend reading some Massad Ayoob.
anyways s&w's new shield is something I'm thinking about too. I hear it has a glock tier trigger. IMO the nano is overpriced. if you don't care about an external safety take a look at the keltec pf9, which can be had for under 300, also tale a look at rugers sr9c, which also has an awesome trigger, an external safety and hold 10 rounds flush and 17 extended which you said you like.