I think you would like 20/4 better. Sounds like the 12 hours darkness and being root bound is causing your plants to stretch. How far apart was your node spacing?
I too try to keep my various plants on the shorter side and use 4" (1 1/2 ltr) pots in a tent under 400whps....I also
run my light 12/12 on all my autos...yeah I know I'm reducing yeild but I grow 5 or 6 at a time and only for myself and to share with special friends so healthy plants are my goals with yeilds being secondary.
That being said, I've had lowlifes, short stuffs, lowryders all got 24" and up with a few staying under the 14-15" size.....
I, for one, have never had a good onyx grow and have over 30 seeds in stock...I had such great hopes for'em that just hasn't panned out like some others I've had...They seem to fatten but during dry, they turn to wispy light buds....good smoke but just not the buds that others grown right beside them under the same conditions have done....My next onyx run will be when weather turns cooler so I can run my tent 20/4 and see if it will change things....Right now, I get my make up air from a crawl space and suppliment that with a diverted ac duct to keep temps controlled and run only at night....