I don't know your gearing of course, but if you go to 13", you will probably lose around six or eight mph on the top end, I believe, but accelerate slightly more quickly through the gears. And also more than likely you will have slightly less rotating mass, unless you get really heavy wheels. Do you approach redline in your top gear often, where the lost mph will be felt? Otherwise, I don't see a downside to going smaller.
It should be noted that I am far from an expert at this kind of thing, but the theory is pretty straightforward.