I have done some testing in this actually with my kits and some through work.. 35w is not as hot as a standard globe, 55w is just slightly hotter, 70 watt is about the same as a 100w halogen globe and a 100 w HID globe is very hot and I probably wouldn't recommend in a plastic Driving light which is why I got the Hella Rallye 4000 which is metal and glass.. I run 55w Hid in my headlights in my troll and they are fine but wouldn't go any higher than that..
35w is a good upgrade for better light, 55w is just bloody brilliant but prepare to get flashed a lot unless you have them aimed very low