I've read "Internet stories" (meaning I've never tried it or experienced it myself) of melted wiring harnesses from using a higher wattage bulb than OEM. It'll draw more current = more heat and the gauge of the wiring isn't designed for it.
Example warning here:
http://store.candlepower.com/48nahiwamola2.htmlAnother problem people experience are plastics might melt with the higher wattage bulb (i.e. plastic reflectors, lenses etc).
Since the 100W is for the high beams and if you don't use it continuously you might be okay, but please don't take my word for it... if it was me I wouldn't do it. Have you researched an HID solution for motorcycles? eg
http://www.webbikeworld.com/t2/hid-motorcycle-driving-light/EDIT: bill who has experience with the modification has had no problems so you should be okay