I did... well not to 15+ but I did the stock 12 to 14.
My friend had a 14mm intake but no carb, and my intake was fucked up anyway, so i milled the 14 down perfectly flat, then just took a carbide dremel bit and widened my carb to match... not that hard just be careful not to go too deep... take it slow and just do a little bit at a time...
then smooth it out a little and make sure it is as round as possible.. remember also better to be a little small than too big.
I'm sure it would probably be better to do this with a drill press or something skillfully i.e. actually "boring" it out... but a dremel works fine if you're careful and look at what you're doing often.
Just FYI... going 12-14 made ALMOST no difference. Seems my ped runs a bit smoother overall but not much of a noticeable power upgrade.. still have to mess around with it a bit though. I dunno if you could make a stock carb any bigger than 15 cause at that point it would be as wide / wider than the hole... but if you have the intake to match and the balls to try it you could prly go to within less than 1mm of the wall... who knows. Good luck!