I used to run a 46x18 which is close to you 39x15. I would top out too easily and bounce around in my saddle with the 46x18 setup. Now I can go a little bit faster and skid/stop still without trying too hard with the 48x17.
Changing a cog is a lot easier and less expensive then changing chainrings. Try a 14t cog, maybe you will like that a little better.