Anything above about 6500k will be worse than the standard bulb, the more blue in the light the less light will be reflected. Although many think it looks better.
The K figures have nothing to do with light output, just the colour of the light. I suspect that as the stock bulb is 4500k and white light is 5000k that your not gonna get any better unless you can get 5000k bulbs.
Colour Temp (K) Colour Description: 3000k Yellow light (suitable for fog lights) 4300k White light with tinges of yellow. 5000K: PURE WHITE LIGHT. 6000K: White light with tinges of blue 8000K: Distinct blue color light 10000K: Bluish-green light 12000K: Bluish-purplish light.
Unfortunately you will never get a low to the ground sports car with shaped headlights to be anywhere near as good as a higher sitting car with headlights sitting at 90deg to the road. It's just not gonna happen with the angles involved.
Funny enough, the Zed is has the best headlights of any car I've owned.