How specific should I be in my SOP, regarding courses and thesis subject?
The expected Statement of Purpose probably differs between fields, countries and even institutions. In economics, it is not a major factor in admissions even if required. People write about their general interest area and proposed topic, with the expectation that their actual thesis may be completely different. 

As I understand it, the purpose of the SOP is just to demonstrate having an idea about the field in general and being able to distinguish subfields. As mentioned, this may vary across fields.