Stick to 4300K to 4700K to remain at least partially legal!!
The very high Kelvin ones tail off into the violet / ultra violet range an give off a lot less visible (usable) light.
Also you WILL get nicked for having VERY blue or purple lamps.
Optimal is about 4300K to 4500K - which is the standard colour fitted to most makes of vehicle eg. BMW, Merc etc. This gives a very intense SLIGHTLY blue tint white, 4700K is a blueish white, 6000K and over is really blue.
The number has NOTHING to do with output - just colour.
I suspect the the dealer is talking crap as 8000K has less visible output than 4300 or 6000!!