I have tested and tried the 21/12 photoperiod, aka Planet Ito by Mister Ito (from overgrow?), and although it sucks up more electricity, I have noticed a difference in yields. I understand that plants stop using the light around 18hrs, but it's the only way to program the digital timer. It's almost impossible to the lights on/off manually, without screwing it up.
You don't need to run for the whole flo cycle, a week or two is enough, but I suppose going longer might push them even more. I've done it with flo's to get pre-flower going, and then move them under a 400w for 12/12. I've done it with the 400, but the heat rises to above 90°F, so it's tough to balance. I would like to try it with a dimmable 600w ballast, running 50% for a couple of weeks (2-3 ito cycles) and then switch back 12/12 at 100% power.