The LEDs must stay cool for maximum efficiency. And when I say cool I mean no more than 30-35 degrees Celsius ( 95F).
In addition, a led efficiency is calculated as the ratio between the emitted light (measured in lumens) and the total power consumption (measured in watts).
The power that in not converted in light is lost as heat. This heat must be dissipated quickly or the LED will fail. This is why so many LED grow lights incorporate fans for active cooling.
Speaking in terms of 'watt per watt', the head produced by a LED panel is much lower than the heat produced by a HPS lamp, but in the same time the LED panel does not emit IR or UV .
To avoid using noisy fans, the LED grow lights can have sometimes over-sized hetsinks to be able to dissipate the heat in the surrounding air.
And the conclusion :
Watt for Watt, say 400w LED vs 400w HPS, does LED produce less heat?
Yes, LED produce less heat than HPS !