A must read is Dan Stern's site on lighting:
Specifically his articles on "blue bulbs":
and the myth of PIAA "SuperWhites":
Please also read his whitepaper to the NHTSA, "Where does the glare come from".
The PDF is here: http://dms3000.dot.gov/docimages/pdf82/176157_web.pdf
Pay close attention to pages 43-45 of 66 pages total. In short, HID retrofits into a headlamp designed for a tungsten/halogen lamp might yield *brighter* light, but too much glare. HID's are an "arc" and as such, need headlamp reflectors/optics optimized for an "arc".
1988 360TE AMG