OEM is 13" as far as I know.
11.5" is about as low as you want to go. As Chris mentioned, you may have some clearance issues going that low. Some say it has an ill effect on handling also.
I had 12.5" progressive 440's and the forks lowered 1" internally and liked that setup. I'd say the biggest downfall of the bike is chassis wiggle. I can feel it squirm under pressure. Shocks, fork springs, intiminators, even solid motor mounts don't seem to get rid of it. I'm hoping that adding a braced swingarm will. Guess I will find out soon enough. Maybe frame braces, if I can ever find them for a decent price.