Technically Kelvin ratings has nothing to do with the brightness of the light. It is the color temp that "K"represents. Your brightness is actually measured in lumen. So in the real world if you were looking for the absolute brightest and MOST usable light then it will be from 4500-5000k. With 6k being the absolute highest I would recommend. Not to bash on you or anything but I just thought you should know.
Here is a link from a forum for you guys to check out.