My take on this is that, well, Godot ways of doing things is already CBA. So 1) There should not be no drop on performance. 2) By dividing things into nodes and attaching them with corresponding gdscripts as you see fit. 3) I don't think so, other than the Godot documentation.
EDIT: answering your comment: yes. Just remember that scripts attached to nodes defines behavior for that node or sub-nodes.