Here is another guy about to go against the grain as far as far as adding a mag extention to your 27, like already said above the 27 is meant for maximum concealment if you want a bigger grip you should look at a 23.
I bought every kind of extention on the market (+1, non +1) and although they did add that extra finger groove I found them to take away from the 27s concealability which was the soul purpose for me getting one.
With a little bit of practice I now shoot my 27 without any extention and find my groups to be improving with every range session, like posted above just curl your pinky under the mag as if you were holding it in place and with a little time this will become second nature.
The one point that I want to make here is that if you want an extention by all means go get one, but do not feel as though you need one in order to shoot the 27 effectively, this gun will do its job with or without any mag extentions, you just have to do yours.
Also, the 27 is one of my most accurate pistols to date. Like Ala Dan I am a big Sig Sauer guy, I can surely appreciate accuracy from a gun. Yet I am simply amazed of the accuracy I get from that little black beast. There is something to be said about a gun the size of the 27 that holds 9 rounds of potent .40 rounds that tucks away with ease and shoots as good as its older brothers the 23/22. Shoot that gun for a while, then post a couple pictures of your targets with your 1" groups or so at 15 yards to show all the "those horrible .40s are so all over the place" people just how all over the place that little .40 can really be
I see many Glocks in your future now, be ready.
Good luck and good choice.