Do a bench test. Back before I got rid of my banshee, I was going to do the same conversion that Jason did. (There was a thread on this somewhere). I did a bench test with a spare set of lights I had... In one light was the PIAA 35w bulb, the other light was the Xentec 35w HID kit. I ran them overnight (bad idea). The PIAA halogen bulb destroyed the housing. Heat rises (temporarily)... I left them with the lenses pointing straight up... came back in the morning, and had a hole clean through the lens on the halogen light. HID was still going strong, and the lens was cool to the touch. It was about 55 degrees in the garage when I checked them in the morning.
Halogens produce around 29 lumens per watt, HID's produce 110 lumens per watt (roughly). A 35watt halogen will be about 1015 lumens, and a 35 watt HID will be 3850. Of course every manufacturer will be different, but that's a pretty good average.
The rest of the wattage eaten up by the halogen, is converted to heat (Waste).
Keep in mind, though. That your stock housing was designed for a halogen bulb, not an HID. You won't get the most efficient light dispersion, but you can see by Jason's pictures, it doesn't really matter... lol. They are BRIGHT, and throw the light VERY far...