14g spokes are pretty much the thickest, and honestly what I am trying to get at is that with tension being equal, I don' think thicker spokes will make your wheel stiffer, they'll just make them heavier. DT does make a butted spoke that is a 13/15/14, and it probably won't fit in your holes, it is usually used for tandems and such. If you look in wheel-building catalogs you may see a reamer for making the holes larger.