Use of the following FAQ requires you to modify (CUT INTO) your halogens enclosures. Replacement enclosures cost $300+ each. If this isn't enough of a deterrent keep reading, but you understand that this is at your own risk and the author is not to blame for any problems that may arise. DO AT YOUR OWN RISK...
Thoughts and Observations
There has been a lot of debate over the years on Audiworld about how well xenon upgrades work in the halogen projectors. Mainly people have been worried about the focal length the bulb must be from the projector lens. Having a set of xenon enclosures in my possession I noticed a few things. First, the US Specified xenon enclosure lenses does not included a fresnel pattern. Second there is a block plate inside the xenon enclosure that does not allow direct light to go to the projector lenses. All the light in a xenon enclosure is reflected inside of the projector. Therefore there is not a defined focal distance in the xenon enclosure, which makes the various concerns a mute point.
I was part of the group buy with Supreme Power Part (http://www.supremempowerparts.com/). They offer xenon kits that work with any bulb type. In the A4's case a xenon conversion for H7 enclosures. Xenonshop is another popular supplier.
My kit included: