I'm not sure about automotive HID lamp. But in general lighting, metal-halide produces more heat than a halogen bulb.
Example:
small 50W halogen bulb (rocket - base GY6.35) bulb wall temp max. is 430C.
35W metal-halide (T type - base G12), OUTER bulb wall temp max. is 450C. the arc-tube temp will be way higher than 450C.
This is because of the high pressure arc-tube.
heat dissipation is not only related solely to power consumption. Types of lamp technology has to be consider as well.
e.g : ur house fluorescent tube 36W or 58W produces very little heat comparing to the metal-halide 35W.
So, in automotive HID, the 35W HID bulb could produces as much heat as the 55W bulb.
btw, HID produces a lot of UV radiation. Cheap bulb has no proper UV filteration. UV could also be a contributing factor to discoloring of the plastic headlamp cover.
just my 2cents