HIDs are efficient, but they have a strong Lamp Lumen Depreciation (LLD =the % output of the lamp relative to initial output after 40% of rated life.)
HIDs might decline to 65% of their original output after 40% rated hours. You'd need to compensate for this accordingly. (rated life might be something like 10,000 to 20,000 hours)
Flourescent lamps have a less pronounced curve, with an LLD of 80-90%, so typically see more constant light output there during their useful life.
There are other factors that will reduce output, including the reflector or element getting dirty over time.
Some references:
http://72.14.203.104/search?q=cache...s&ct=clnk&cd=20
http://www.energy.gov.on.ca/english...ghtingGuide.pdf
A big lighting guide PDF - search on LLD