"HID kits" in halogen-bulb headlamps do not work safely or effectively, which is why they are illegal. See
here. This is the case whether you have a halogen reflector headlamp or a halogen projector headlamp — and no amount of marketing hype or uninformed "Yeah, looks okay to me, I love it, it's great" chatter on the internet can change that.
Silver Stars and all other blue-glass "extra white" bulbs are not the way to see better. All they do is tint the light, they don't intensify it. The debate over this immutable fact comes from advertising hype on the one hand, and the fact that "brightness" is a subjective impression that means nothing in terms of how much light (or how much usable light) is present; the one and only valid measure of that is
intensity. It's like "loudness", which is the same kind of subjective impression with regard to sound (in that case the only valid measure is
sound pressure level).
The Osram Night Breaker already linked is the highest-output H11 presently available. If you want more than that, you can usually without much of any difficulty replace the H11 (around 1300 lumens, nominal) with an H9 (around 2100 lumens, nominal) like
this one. Unlike an "HID kit", this swap works safely in headlamps like the ones on your xB because the filament size, shape, orientation and placement are all the same in H11 vs. H9. You may have to shave a small ridge of plastic off the H9 bulb's connector so your H11 socket will snap on. Other than that, it's a direct swap.
Which way is most effective? H9. You get more light.
Which way is most cost-effective? Probably H9 there, too. The H11 is a popular choice for low beams because (among other factors) of its very long life, but this comes at the expense of filament luminance and luminous flux (and resultant headlamp beam intensity). Night Breakers have higher luminance and somewhat higher luminous flux, but cost a lot and have a relatively short lifespan similar to that of the standard, ordinary H9. That plain H9 has very high luminance and flux, much higher than any H11, and doesn't cost a lot. So whichever way you go, you're looking at shorter bulb life, but you would get a great deal more light with an H9 swap.
There's supposed to be a base keying difference that prevents the interchange of different bulb types (like H9 in place of H11) but in fact the base keying isn't different enough to prevent the swap, it's just enough to require you to jiggle and wiggle the H9 into the H11 holder.
