Although I have not personally tried modifying the 12/12 in bloom, from my reading up on this topic a couple months back it seems there is a negative correlation to THC potency the more you deviate from the 12/12 cycle (both ways). There is also a debate as to whether reduction in the photo period will result in earlier maturation and reduced flowering time - at the expense of size and potency. This may be strain dependent. I have not found a conclusive study on this, but I would guess there is some truth here. Outside the debated shorter maturation time, most folks likely will not go with the shorter photo period because the tradeoff does not make much sense.