There have been instances where the McEwans yeasts (1728, wlp028) have gone past 16% which doesn't surprise me. It cut it's teeth on wee heavies from way back, there were/are some 20 odd strains in the culture. I personally wouldn't worry too much about it as long as they're healthy and it sounds like they are, see where it takes you and go from there.
Edit: You could probably add a little yeast nutrient to be certain.
Newer, better, more streamlined sig as per the forum police.