I don't transplant autoflowering cannabis at all. They go right to the final pot.
The thing to remember about "super-soil" is that there is practically no such thing unless you make it yourself. At the store, you have potting soil and then you have organic potting soil. Neither contains a substantial amount of phosphorus, mostly because the vegetative phase doesn't need that much phosphorus. Another reason why potting soil doesn't have much phosphorus is because fertilizer runoff is an ecological hazard, even when the fertilizer is organic. At some point your plant may demand more phosphorus, or it will look as though it's not quite reaching its full potential. E.B. Stone's
Sure Start and
Recipe 420 Ultra Bloom fertilizers are great options for phosphorus fertilizer.
Other things that may result in stunted growth are inadequate light and a contaminated water supply. Autoflowering cannabis needs a lot of light, which is why professional growers dislike them despite harvesting flowers sooner, and less hours mean less growth. As for contaminated water, the most common contaminations in municipal water are chloramines which do not evaporate like chlorine. These chloramines and other potential contaminations can be filtered out using a reliable two-stage filter, such as the
Camco Hydro Life Premium Dual Water Filtration System.