Probably irrelevant to the current enquiry but worth noting is that LED technology is advancing at a rapid pace and very soon it seems possible that LED headlights will supersede HIDs and Incandescent filament bulbs. The Audi R8 already has them, and the new Seat Leon has an option to fit them. Again they can be tuned to give a wide variety of colour temperature and use comparatively small currents without the need for step-up transformers and give instant light without the warm-up time of HIDs. Still quite new technology but destined to take over eventually.
Also be aware that some filament lamps use Xenon gas in the envelope to enable the filament to run hotter (and brighter) without premature failure. Putting the name "Xenon" on the bulb packaging no doubt adds confusion although it is an accurate description. Xenon does not always imply HID.
HIDs are generally not suitable for main beam use. As was already stated, they do not react fast enough to be flashed and there is a small but perceptible warm-up time. In addition, the greatest wear on an HID bulb is at start up and particularly on restart with a hot bulb, leading to premature failure.
In terms of colour temperature, brighter is not always better. As the colour moves towards the blue end of the spectrum the visible light reduces. From a subjective point of view it is easy to confuse whiteness with lamp output and you may end up with a lamp that looks whiter (or bluer) but is actually putting out less light than a "yellower" bulb. Summer daylight is generally accepted to be 5500 Kelvin fading to 4500 Kelvin by late afternoon and for this reason OEM lamps prefer to stick at around 4800 Kelvin to give a good balance of whiteness and output. By the time you get to 8000 Kelvin lamps (Barry Boy territory) you have noticeably blue lamps but much less output, to the point where incandescent bulbs are possibly brighter.
In terms of legality of HID lighting systems there is as usual a huge amount of misinformation and interpretation in circulation. The VOSA document is here >> http://www.partinfo.co.uk/files/MOT%...20Lighting.pdf
To distill the words -
1. If your car has stiff suspension (what constitutes "stiff" is not stated), then no self levelling is required.
2. If your car does NOT have stiff suspension, then it needs self levelling of the lamps or self levelling suspension. The manual levelling adjustment on the dashboard does NOT comply with this directive although in practice many MoT stations won't realise this.
3. If you car has HIDs then by definition it NEEDS a headlamp washing system even if one is not fitted as original equipment. For an MoT pass on a car with HIDs under VOSA regulations, a washing system MUST be fitted and MUST be working. This is one of the most misunderstood clauses of the revised MoT and one which seems to the most ignored, but if your car fails on the grounds of no washing system, at least you know its your fault. If you then try a different MoT station it is likely they will have a different interpretation and you would get a pass but the regulation is clear and specific. The likelihood is, and the VOSA recommendation is, that you would get an advisory where no washing equipment is fitted and its inclusion is in debate.
In terms of the Construction and Use Regulations, the Department for Transport guidelines of 2010 still apply and "The headlamp unit (outer lens, reflector, bulb) shall be type approved to ECE 98 and be "e-marked" to demonstrate this. That can only be done by the headlamp supplier - Hella, Valeo etc. - who must test the headlamp in an independent laboratory." To be honest I have never checked to see what lamps are e marked and which are not - and I doubt whether anyone else does this either.
Further to that, the DfT consider "The use or sale of aftermarket Xenon HID conversion kits is not permitted and would contravene Construction and Use regulations. Only approved bulbs can be fitted to vehicles used on UK roads. Converting a Halogen Headlamp unit to a Xenon HID Headlamp unit using a Xenon HID conversion kit is not legal and cannot be approved."
Note they do not make any distinction for projector type headlamps as fitted to 159/Brera which have been proven to give an acceptable beam pattern.
Through all this legislation, many vehicles continue to pass MoTs with HID lamps