If you can get the DLI up to about 40, I dont think you gain anything but a higher electric bill for growing them under more the 12/12. It seems like pouring 5 gallons of water in a 3 gallon bucket. Your grow though 

I am never in that big of a hurry for plants to finish but I can tell you for sure, dont grow a Landrace Sativa. I have a Jamican going that has been under 12/12 for 6 weeks and it is just showing good clear signs of flower. I saw enough signs of flower to know it was a female 5 weeks ago.