Computers are well over 99% efficient as heaters. The only energy they emit is a tiny amount of EM from LEDs, clock signal and the transformer, and a minuscule amount of fan noise (which is almost all absorbed by the immediate environment anyway). Electric heaters similarly emit EM and noise, so I wouldn't be surprised if some are actually less efficient as heaters than the average computer.

It is possible for a heater to be over 100% efficient by removing heat from an external environment, but heat pumps require significant supporting infrastructure.