Here is an interesting article comparing 35 watt HIDs with 50watt. Its actually for conversion kits for light aircraft landing lights and they test 4 or 5 35 watt kits and one 50 watt kit.
http://www.preciseflight.com/ufiles/06March_Aviation_Consumer_HID_Article_Main.pdf
All the 35 watt kits performed similarly, but the 50 watt HID stood out notably.
"The real stunner was XeVision’s 50-watt lamp, whose brightness is best described as stupefying. Our target holder squinted in discomfort as he walked downrange to collect intensity data with the light meter. The 50-watt HID delivered more light at 600 feet than our stock light did at 200 feet."