There is no such thing as a boiler that operates at 100% efficiency. At 100% efficiency, the boiler would transfer all of the heating energy units (B.T.U.ís) contained in the fuel, into heat or hot water.
Boilers are cleaned because a clean boiler, as contrasted with a dirty and soot laden boiler, costs less to operate and maintain.
A clean boiler uses less fuel.
Boiler repairs are smaller and less frequent.
Air pollution is minimized.
Minimize the risk of mid-winter boiler shut downs.
Potential fire hazards are eliminated.
Summary: A Boiler Cleaning may cost some money, but a dirty boiler costs more, much more!