Typically engines are more fuel efficient when the power is pulled back and you get closer to some efficient L/D....
Most engines are so overbuilt for the job at hand where it will put out X power, but they only give you Y(derated) and the operator only let's you push it to Z (reduced) trying to stretch the life of the engines out indefinately..
Is it possible that some engines are loping along as such reduced power they aren't efficient? Sure....which ones? I don't know...but comes to mind putting a 454 in a Volkswagen Bug is a pretty inefficient use of power, milage sucks, but the engine certainly won't break pushing that little car around...
Personally I would take a lighter engine, less overbuilt, running in a more efficient operating range to save some fuel...if the fuel savings were such it made sense...but not if engines were breaking left and right...
I guess the question is the model of how much over building of engines you need that once powered back you get to some calculation of reliabilty where the engine will last for x amount of hours/years ect....