For what it's worth I've just done quite a bit of desk research on HID kits. These were some of my (not always conclusive) findings. It's a long post, but you might find something useful in it:
Not all kits are the same. Some manufacturers of kits mainly design build and test the electronics and don't have much to do with the lamp unit itself. It's the lamp unit that finally determines your beam.
All lamp units sold in kits to replace H series lamps come with non certified lamps. They have to, because the lamps and possibly their original holders are made for oem use and need to be adapted. It's easy to certify/approve the electronics.
There are 2 'reputable' sources for hid lamps fitted as oem in VW, BMW, Audi etc - Philips and Osram. You can buy their lamps in D2R D2S bases NOT H series and they are fully tested and qualified. Most low cost kits will be using Korean lamps. I read mixed reviews, so it's important to find a kit where the supplier goes to a lot of trouble in their testing - and ask questions about lamp testing in its holder! Oem units sell for around $1000 and a big chunk is Environmental/EMC/QA testing for uniform conformity.
Electronics in 2 parts (ballast & ignitor) are generally older design and more to mess around with in the limited space of our bikes. I think the ballasts incorporating the ignitor (smaller package) use more sophistocated design to start the lamp quickly and maintain arc brightness.
The precision positioning of the arc in the lamp, in relation to its modified base is not always guaranteed, tested or specified. The generic light output figures (300%) for hids are the lamp capsule only, not when mounted in the reflector as you use it.
All hid capsules age to bluer, higher K, lower light output. The gas mix and lamp quality will determine how fast a capsule will age.
The ability to cold start and reach full brightness quickly can vary widely across different kits, particularly where a dealer is only assembling components and badging as his kits. Delay in starting and reaching full brightness is important on a high beam.
Some kits have proprietary lamps and only work with the suppliers lamps. OK the life is long, but it's better to find out if another lamp can be fitted some time in the future or the price of a replacement lamp.
Vehicle lighting regs vary country to country. The kits are not certified and use a loophole as aftermarket replacements in some regions, so check with your bike test station first.
Lamp starting and running at low temperatures can be very important.
Restarting a hot lamp is generally fast but worth checking.
Most kits now are sold with lamps which incorporate a UV cut off filter. Our bikes have expensive plastic lenses so check what you buy incorporates a UV filter.
Some ballast units are not tested to be waterproof. generally this is because they are sold for the cagers where there's plenty of room to get the electronics out of the weather - not so on our bikes.
Some kits are not easy to retrofit back to filament - e.g you are on a long ride and the lamp or ballast fails.
The length of leads from the ballast to the lamp capsule can vary from short to long.
Some ballasts do not have E specs for interference - important if you are running Satnav, radio, radar detector etc
The higher the color temperature the lower the light output and bluer the light. I think 6K is about right to stand out more in daylight.
Warranty is important, the ballasts are high voltage units and need to be very rugged. Some units are made in ABS boxes, better ones are aluminium.
Some ballast units do not have fixing points on the actual case, they use a spring clip you fix separately to the bike/cage.
Hid installs can leave the low beam a couple of degrees to high, you find out what you've bought when you get your lamps put on a beam tester, checked and adjusted.
What to do with the parking bulb? Nice to have something blueish to match - will probably use a Philips extreme white or blue halogen.
Brighter hids produce more light scatter if you don't keep the front lens clean. Some regulations on new vehicle builds require auto levelling and headlamp washers to be fitted.
There are plenty of cheap kits on Ebay. After what I learned, I'd be wary and rather spend a bit more to get a kit with some testing, specs and which other bikers have given plenty of feedback on. I wouldn't put a bulk purchase deal together until I'd gone through the above and road tested one, but without decent testing one might be ok.
On the other hand, a really cheap kit might be worth taking a chance and be a good buy if it works for you.
I won't feedback which kit I'm looking at or trying until it's been on the bike a few months. I want a reliable warranted unit with good headlamp aim, easy change back to filament, able to use lamps from other manufacturers at a push and don't mind paying a bit more.
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