Hey guys, I've read through this thread and I think I have a few words to throw in...
I've noticed the same droopage with my plants be it Photo or Auto, It's like they have little clocks built in them.
Even though an Auto is not photosensitive they still can be shocked by sudden changes in atmosphere and light regime.
From my own lessons I would have to say ounce a plant is in full veg or flower it is not wise to change anything as important as light regime.
I know that not everyone one has this available to them but when I was working my Thai Haze #47 (F4) I wanted to know what the Optimal light regime would be for them because I was very close to releasing them. So, One closet was 18/6 one closet was 20/4 and the final closet was 24/0 but outside the time of year was 12/12. Plants were growing in all 4 light regiments. For my Thai Haze the optimal light regime was 20/4. Each 20/ plant would yield up to an OZ more than the other plants with different regimes... But that is just one strain and my own learnings with that one strain but From those lessons I got higher yields out of any Auto I grew.
Also temp. is a major factor with every aspect of growth.
Just my 2 cents, Be green ya'll