For some reason turbos seem to help efficiency on diesels, but make it worse on gassers as compared to normally aspirated. I haven't quite thought that one through yet.
Even the
Rutan Voyager that flew nonstop around the world was normally aspirated. It also was essentially true multi displacement, with a 200 in back running for the whole flight and a 240 up front to help w/takeoff and initial flight. I haven't seen any cars try the multi-IC approach yet.