Actually a TRUE HID (D2R Xenarc) capsule w/ hi voltage ballast power supply draws less than a filament bulb. Yes, when a HID capsule first kicks off from stone cold the current draw is substantial for a few seconds. Then the current falls off as the pressure inside the bulb builds. Once the bulb has reached it's HI pressure, the current falls way off usually to less than an Iodine filament bulb. The light energy produced by HID is a function of the capsule pressure and not current draw. I have done conversions on cars and I can tell you from experience it is not just a simple bulb swap. Far from it. A lot of design goes into the lens and enclosure to prevent glare. It can be a nightmare if the original bulb filament was horizontal. Having an enclosure that was used with a longitudinal filament (one that was front to back) does ease the problems but you still have to correct focal point issues.
I know what your saying that this guy is crazy but trust me. I would not do it again unless the enclosure was Designed for a HID capsule. Sure you can just install a bulb in a standard fixture and ride, but from that moment on everyone will be blinking their Hi beams at you. You will also have Hot spots on the ground where the light is concentrated and areas where there will be shadows.
Now, if you are using an enclosure that was designed for a D2R Xenarc Capsule, go for it. You will love the even white light blanket you get. You will not have any current load issues. That is the least of your problems. It is truly wonderful technology.