What would happen if the efficiency was 100%?

Such a device would contradict all the laws of physics, because the efficiency exceeding 100% is a classic “perpetual motion machine” that refutes the second law of thermodynamics (which establishes the existence of entropy). If this law did not exist, then heat engines (refrigerators, heat pumps) simply would not be able to work, moreover, the entire Universe would function differently. So reasoning about the advantages of using a super-efficient engine with an efficiency of more than 100% is pure utopia and one of the favorite tricks of pseudo-inventors who take advantage of the naivete of gullible citizens.