I haven't done the HID conversion but I have pondered it. The advantages of course are the higher light output, but the disadvantages outweigh that in my opinion.
The major disadvantage on an XL is the necessary Dc conversion to the headlight. I have read stories of lights shutting off due to voltage and/or battery reserves being low, and an XL is marginal that way anyhow since the DC circuitry is not utilized or designed very heavy. Bulb expense and vibration resistance fears compound my reluctance.
Maybe as an auxiliary light but there I would probably opt for LED driving lights instead keeping in mind current draw limitations.