I know this is an old post but i tried with 11.5 hr of light and 12.5 hr dark, and it did speed up the flowering process as normally first signs of pre-flower (using normal 12/12) takes around 14/16 days to show, but using 11.5/12.5 pre-flower showed on 10/11 day, using two differnt strains both showed sex on the 10/11 th day.
i cant comment on what the final results are yet, i think it will be faster to grow but loose benifit of that little extra light to feed up the plant on energy to grow me some big fat a$$ budz