well man throwing in HID's is not the same as throwing in regular bulbs!!! The hotspot of the bulb is NOT going to be in the same place as the hotspot of your halogen bulb, thus you'll get spotty lighting/glare/bad output!!!! Remember, engineers put in countless hours to desing a HID housing. Throwing a HID bulb in a halogen housing will not always be beneficial.
The things you could do, or will have to do,,is modify the base of the bulb, or create a glare shield.
i suggest going to the philips lighting website and go to their lighting forum. Believe me, you'll learn a shitload there.
hope i could help