This mod appears to have extended the growing period but not increased the harvest quantity.

I find the idea of this mod very intriguing and realistic, however being dependient on the 90 days seasons mod really hinders the experience due to lack of balance. It would be better if there was a version aimed at the vanilla season length, otherwise it needs such a comprehensive overhaul of the entire game that I don't think it's feasible with mods yet.
As it stands, when i tried it I ended up capped on population, with the entire tech tree unlocked and with my castle 100% built in the very first days of the first summer, without any harvest, at which point the map had nothing else to offer me in term of gameplay and I abandoned the fort.