There was a controlled experiments: clones for same mother, grown at 12/12 and 10/14. The THC content of the 10/14 was 50% less than using 12/12.

I ve seen botanic studies wich show that more hours of light not only report higher yield (by more total iiradiance), but it affect to how the growth is distributed: more hours produce more carbon fixation to flowers and less to leaves and stems.

I upload a pdf of a botany study about (performed with similar plants to cannabis in photoperiod responses).