Sorry but to stop false information spreading I must again point out that headlight washers are
not there for the purposes of cooling the headlights.
1) They are there for removal of dirt from the headlight to stop/reduce dazzling from built up dirt on the headlights, as stipulated by EU regulations for the production of new vehicles.
2) They are only operated when you operate the windscreen washers, and even then I think it's only every 2-3 pulls rather than every single time. So unless you drive around while constantly pulling your headlight stalk backwards they are not being "cooled" anyway. You don't need a physics pHD (which I have!!
..not got) to work that out.
From HIDPlanet (who are like the kings when it comes to all things Xenons)
The tungsten filament in a halogen bulb burns at around 2800°C during standard operating conditions.
The arc in an HID bulb burns between 2000-3000°C depending on the manufacturer and generation of bulb.
That is the actual BURNING temperature of the core part of the bulb that burns to produce light
Further to that:
3) I had Xenons in Hella Celis projector headlights for 18 months, I didn't have washers, and never once did I burn or melt my headlights in any way..
I'm sure the places you read that Factory Xenons came with washers was to cool the headlight, are the same places than says diesels with a decat will fail the MOT on emissions!
The reason that VW likely do not advise using headlight covers for Xenon lights is because of the fact that the beam pattern would not be correct.
Also that they have the tab on the inside of the headlights to change them...