The light output on HIDs isn't exactly linear, it's a bell curve with 4500k-5000k being the brightest. Anything else won't be as bright, going both towards purple and yellow.
I chose 8000k because, from what I could gather, the light output is very close to the same as the stock halogens. Meanwhile the halogens were 55w and the HIDs I got are 35w. So I am drawing less power for a light solution that should last longer. I decided to get HIDs in the first place because one of my halogens burnt out.
For the same light output, I consider the light scatter/cut-off to be acceptable. If I wanted to pump out as much light as possible, I would do a full retrofit.