"Colour temperature ('K') vs brightness...It is a common misconception that higher colour temperatures (K’s) produce brighter lights. This is not true. The colour temperature only determines the colour but not the brightness of the HID light. The Colour Temperature is simply a scale represented by the Kelvin Temperature Chart (hence the abbreviation “K” or “K’s”) as decpited above that measures the colour of the light output. Typically, the higher the colour temperature, the closer you get to achieving bluish to purplish light colours."

and

"The higher the Kelvin, the less light output you get (lumens).
With that said, anything over 6000K is basically a waste.
So, what is the best bulb? IMO the 4300K is the best, as it has the highest light output. The problem, though, is that they have a yellow-ish tinge to them that some people find un-attractive. In that case, 5000-6000k is a better choice for you..... as they have a more blue look to them."


Here is a good chart



Some other good info from this site.

And a comparison of Kelvin to lumens;
Standard OEM halogen 55W 9006(HB4) = 1100lm (lumens)

4300k D2S Philips = 3200lm (lumens)
4300k D2R Philips = 2800lm (lumens)
4300k D2S Philips = 2400lm (lumens) actually 5800k
4300k D2R Philips = 2000lm (lumens) actually 5800k
4800k D4S/R (brand) = 3800 (lumens) ** brightest in the market
5800k D4S/R (brand) = 3300 (lumens)
7000k D2S other = 1790lm (lumens) *(other bulb brand)
7000k D2R other = 1390lm (lumens) *(other bulb brand)
8000k D2S other = 1180lm (lumens) *(other bulb brand)
8000k D2R other = 780lm (lumens) *(other bulb brand)

Higher than 8000k, the light output significantly drops off, causing the light to be almost useless.