Geez, you just made me realize I've never gotten around to driving my XF at night. That's one way to extend the life on those nasty bulbs.
On a sour note, I've read several posts on the web that the Sylvania Silverstars burn out quite a bit quicker than plain jane H7's. Just Google Silverstar longevity. But if you save your receipt people have gotten replacements from Sylvania.