What about the stereo are you dissatisfied with and are you sure a sub will fix it? Have you tried mucking about with the EQ? Seems to be some good suggestions in this thread, but I haven't tried them on my 2015:
http://www.ft86club.com/forums/showthread.php?t=15696
I am far from an expert, but in my experience adding a sub to the OEM sound system just makes the bass overpower everything else. It doesn't really improve anything, unless thumping bass is your thing. I had a 2012 WRX with OEM sub. Even with the bass at -7 and the audio type changed from wagon, there were some songs I couldn't listen to.