I don't follow an HID (or any current draw less than the starter) would cause a battery issue, but there may be something I'm not aware of.
But as for the operating wattage/current, that's not the whole picture.
http://en.wikipedia.org/wiki/Inrush_current
Older systems had a lot more than the newer 'slow start' ballasts, but the startup current draw is still several times the warmed-up operating current. Notice the amperages listed on this DDM tuning unit:
http://www.ddmtuning.com/content/tec...llastspecs.pdf
3.2A operating vs. 15A max