It really depends upon which type of HID lighting system you are talking about.
As you go up through the HID Index as in:
Sodium> Mercury Vapour>Metal Halide.
The losses get reduced, but it is at the cost of higher strike voltages.
Personally, if you were going for lowest loss as far as ballasts were concerned, look at the new T5 Fluorescent tubes, they use High Frequency Ballasts and they also are taking on High Bay Metal-Halide and Mercury Vapour in terms of Light Quality and Energy Efficiency.