Not empirical because there's no money to fund such studies, but: Mandatory veganism would almost certainly significantly reduce the healthcare (and food) costs for a society by a large fraction. Banning cars would make the air cleaner, remove a bunch of health problems, and facilitate more social forms of travel which would help form a more cohesive society and probably would reduce a lot of problems which arise from modern urban isolation.

I'd personally like to live in a society where people were mostly vegan, there were no cars, and there were no firearms.

Whether or not we should force that is another question, but it sure sounds nice.