the plant would still Veg in 12/6, and i would bet that the roots would become sickly and weak as you are basically speeding up the plants light/dark cycle from a 24hr period to an 18hr cycle, which also equals major stressing on your plants, thus increasing the likelihood that they will turn out male.

to truly take advantage of a 12/6 cycle, you would need plants that have had some kind of genetic engineering done since the light cycle is programmed into the plants through eons of evolution.