Technically radiation cooling is 100% efficient. And remarkably effective, you can cool an inert object to the temperature of the CMBR (4K) without doing anything at all. However it is rather slow and works best if there's no nearby planets or stars.

Fun fact though, make your radiator hotter and you can dump just as much if not more energy then you would typically via convective cooling. At 1400C (just below the melting point of steel) you can shed 450kW of heat per square meter, all you need is a really fancy heat pump!

How much power would a square meter at 1400C shed from convection?