My 50 watt mr16 home made metal casing light will boil water right off it. I had to paint it with heat paint.
It can melt plastic easy.
HID lights put out more light with less wattage. My 16 watt HID is as bright as a 65 watt halogen light. But it's only 16 watts so it is not as hot. It's about the same heat as a 16 watt halogen light. It won't be as hot as your 20w bulb. I don't think I can burn my hand on the housing, not sure. The front of the bulb is pretty hot. I don't know if I can burn my finger on the bulb, but it feels like it.
The HID housing does get hot but my HID is not supposed to be operated without air moving over it anyway.
Most powerful bike lights say not to operate them standing still, and get hot enough to hurt you.