I think everyone here is assuming that there is no boost without positive pressure. Just because a turbo'd engine cruises with vacuum doesn't mean it isn't seeing boost. Say a NA engine cruised with 18" of vac. Now put a turbo on it under the same conditions but now it cruises with 17". The engine is now running with whatever 1" equals for boost. With a NA type part throttle tune you have now just leaned it out at cruise. Maybe reaching here a bit but I think technically there is a case to be made for better fuel efficiency.
