not recommended 14 is the lowest you should go! someone will be along to tell you why soon, i would expect something to do with the strain it places on the the chain
dont know about that but I think if you go to a 13 on the front it will start to wear the front of the actual swing arm out as the chain will end up too close!!
I run my bike a 999S with a 13 and it was fine, but when I did the chain I went up on the rear and went back to a 14 on the front, what I think is you can run a 13 but do the back if you can, I did not care about the chain as I would have to buy a new one if I was to go up 3 teeth on the rear anyway, so the wear was not a problem,my chain lasted 10'000 and 7'000 was with 13 on the front,