Lots of questions in there. Kinda jumbled, so I'll just toss out the info and you can take from it what you will.
Projector lights for GMT400's - nothing you can buy aftermarket is going to be any good. They're all cheap with very poor optics.
HID's in these things usually come out with a cutoff line, but sh!te output and a narrow, narrow crappy beam. Here's a set done just like that:
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What they ARE good for is a starting point for a proper retrofit projector project.
I did just this. I swapped a set of Acura TSX projectors in with D2S capsules. None of this just drops in. There is considerable fabrication required. I chose the TSX projectors because they are very high quality and relatively small for projector units. This is important as there isn't a lot of room in the GMT400 front end behind the lights. I still had to cut the rad panel for clearance.
But done correctly, the beam looks like this:
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There are other projectors you can buy aftermarket to retrofit, Morimoto housings and ballasts seem to get decent reviews over on HID planet, although I have never personally used them. It might have something to do with site sponsorship too.
You can run HID's for high and low. The issue is the warm up time when you go from high to low. If you are running down a dark road, it's a pretty nervous 3-5 seconds before they get up to temperature. I run HID highs and lows, but I have built my own wiring harness that keeps the low beams on as long as the dash switch is on. The multifunction switch just turns the High beams on and off.
However, if the low beams are built correctly, Hi beams are nearly useless anyways. I just dropped in a re-based HID capsule and use it like that. All it is really good for is lighting up road signs a little further down the road above the TSX cutoff line. This is not my pic, but it is what I see at night when the HID lows are on:
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Mine just look like a set of regular aftermarket lights:
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They weren't chosen for looks. They were chosen for the clear cover (needs to be optically neutral) and the ease of fitting the TSX projectors. for all intents and purposes, they were otherwise gutted completely. I might have used the chrome style ones if I were to do it again.
just for fun, I looked at the "lenses" that came out of the aftermarket housings. Crap. Optically flawed. They were "projector lenses" only in the sense they were made of glass and were shaped to look convex. They were even different thicknesses left to right. Crap. Crap. Crap. Worse than using the yellowed OEM halogens.
Retrofits are a lot of work, require a bit of skill to fabricate and they aren't cheap once all said and done. But the rewards are sure worth the effort. Especially if you do a lot of highway running in remote locations (with lots of Moose!) like I often have to do.
Last point: if you want the best light output, you want 4300K capsules or as close as you can get to them. The Higher K (getting blue to purple) the lower the light output.