I play as Venice, which is not part of the Holy Roman Empire (HRE), but is surrounded by provinces that are. Conquering those provinces will not remove them from the HRE, but you can do it manually.
I know it will "upset the Emperor", but not by how much. If I remove several provinces, will the negative opinion modifier stack? Will other member states care? What about non-members?
Will it affect local rebels?
Also, I guess it will remove the "unlawful imperial territory" event, but that only fires on provinces which are not cored anyway.
What are the pros and cons from removing a province from the HRE?