mvalenz, that has been discussed so many times here. You can do it but you can expect some drop in yield and potency. THC is produced as a reaction to the intensity and duration of your lights. Granted, only so much THC can be produced as the genetic potential of the plant is the limiting factor, all other things being equal. But you will loose some yield and potency. How much? Who knows, it is impossible to calculate as each strain will react differently and has different genetic potential.
That being said, I have ran one strain 12/12 with my photos and was pleasantly surprised. The buds were fairly small compared to the same strain under 20/4 but the potency was still pretty good. So, if you don't mind a drop they will still treat you right.