Its true, your dealing with the nature of the human eyes, as much as you might want to beat around it, night vision and high K rated light do not go together. Just because it seems brighter doesn't mean you can see.
And as far as blinding other people, man I wish I had brights I could shine backwards are the jerk behind me. While blinding oncoming traffic is always bad, headlights are designed to shine more light to the right so that it lights up road signs and anything that might be off the side of the road, and not into oncoming traffic. But when you have someone in front of you, they get the full brunt of it shining of the mirrors. Sure you can dim the center mirror, but the other two leave you helpless, and it hurts the night vision of the guy in front of you.
If you are going to use HIDs, please, use the projectors, they were invented for a reason.
While it is true that a lower Kelvin rating does not automatically equal brighter light. The 4300K bulbs, in terms of lumens, are about 15% brighter than the 8000K bulbs. Color temperature does not equal brightness. Lumens is how actual brightness is measured. Its like those stupid Silverstar halogens which are said to be so bright, if you compare them in lumens to an Xtravision, which has no tinted coating, the Xtravision is brighter, again by around 15%. The Silverstar looks brighter because its apparent color temperature is higher, a result achieved by removing some of the yellow spectrum light.