Light effiency or efficacy and watt to heat output
Ok, so as a rule:
$$Watts_{in} = Watts_{out}$$
This is always true unless you're storing energy somewhere.

A 400W heater will produce 400W of thermal energy.*

A 400W incandescent will produce 400W of thermal energy and light energy combined. Unfortunately if you measure the amount of light energy being produced, it is rather small compared to the total energy consumption of 400W.

A 400W LED fixture will produce 400W of thermal energy and light energy combined, but the amount of light energy is going to be a more significant figure in the process. Now you will be able to tell that it's not 400W being lost to the heat sink, but some amount less than that. (This ratio is the efficiency). The rest is being emitted as mostly-visible light.

However, if you put the LED in a well-insulated black box, you would not be able to tell the difference between a 400W lightbulb and a 400W LED by the temperature of the box.

In general, the actual efficiency of a light source (in Watts emitted as light) is not published.

Efficacy is not the same as efficiency. It takes into account the human eye response that determines how bright the light is, as produced from the source. This is not in units of Watts/Watt like efficiency, but in Lumens/Watt.

For example, comparing a white LED to a UV or deep blue LED, the white LED would have much higher efficacy compared with the deep blue LED, even if the amount of energy released in photons is the same, because the human eye does not respond to deep blue very well.

*As an aside, a 400W heat pump can present more than 400W of thermal power at its output, but it does that by moving heat rather than purely producing it.