The short answer for now is it's all in the optics. Stock tahoe lights have crappy reflectors for HID's. Some reflectors work better than others at controlling the output light, and indeed some are even designed for HID's.
When it comes to seeing the road, the color plays a big role and is why I hate the blue tinted lights--they look cool and can be bright but they don't illuminate the road all that well, and if it's wet or rainy they are even worse. I always run 4500K or less because they provide the best and most usable light. There's a lot of science, light wavelengths that the human eye can see better than others, etc that play a major role.
I'll add more details later when I'm not on my cell phone...lol.
Continued...
Those Q45 headlights, if they are the gattling gun style, are actually one big projector with a bunch of small lenses instead of one big one. Really funky looking, not sure what the point of that is. But I digress....
One of the more fundamental differences in projectors (there are projectors for both HID and halogen bulbs) is that halogen projectors tend to focus and magnify the light in front of the vehicle, while HID projectors tend to focus and spread the light in a wide pattern, giving you a usable beam width and depth that can't be matched by a halogen bulb. I have usable light that extends a couple of lanes to each side of my truck. See the following picture for how wide the beam is. Then try this with a halogen light and compare.
There are very few HID reflector headlights out there. Those reflectors designed to control, focus, and provide a good cutoff. Cutoff is the biggest issue with reflectors, because most of them have no mechanism for this, relying solely on headlight adjustment. This is what keeps all of your light out of on-coming traffic's eyes. HID projectors have a cutoff shield, which provides sharp, distinct cutoff with virtually no light spill above the line.
DEPO's are often recommended because they have above average cutoff and spread for a cheap aftermarket light. They are not designed for HID's, though, and instead have halogen projectors. In essence, the projectors that are there, and the same can be said of all aftermarket projectors for these trucks, are made for a much weaker light source. I'd be interested to see a shot of the DEPO's without HID's, actually. The cutoff and brightness would be interesting, and a benchmark for comparison.
Bottom line is a typical Tahoe stock reflector with an HID capsule just blasts light all over the place. Yes, it might be brighter on the road, but it's brighter EVERYWHERE, including other people's eyes. The point of good projectors is to keep the light bright but out of other's eyes.