Need more info.
For photos, side by side studies have been done where the plants that were vegged, then flipped to 12/12 did better in the same or less time than those that were run on 12/12 from seed.
Autos don't have a photo-trigger, they are daylight neutral. So, less light = less bud. There are plenty of members with photo grows on here where they grew autos right next to them in 12/12. Smaller plants than under 20/4.