Food for thought........
I upgraded the factory HIDs on my DD from 4300K bulbs to 6000K. In doing my research, several sources said not to exceed 6000k because, although the 8000K light is brighter, it has a shorter effective distance. Basically, other people will see you better, but you won't see out better. I will admit that even with the increase I did, I feel more compelled to use my fogs more on dark roads. I just don't feel like I can see as well as I did before the "upgrade". I really like how they look though, and I have my fog/driving lights (which are just as bright as headlights) to make up the difference.
I do know this: Different light sources produce different wavelengths of light, and lower frequency waves travel further than high frequency waves. My guess is that the higher temp of the lights produces higher frequency (visibly brighter) wavelengths, but their effective area is decreased. IMO, the best middle ground is 5000K-6000K.