The 4300K color of the factory HIDs is already as white as you're going to get. 5000K is still very white with just a hint of blue, 6000K is starting to get a definite blue tint, anything beyond that is just foolish. As sj90 pointed out, the higher the color temperature, the lower the actual usable light output. If you want best performance, stay with the factory 4300K. If you're just looking for appearance, do whatever you want.
2011 Pony Pkg. convertible M6
Am WS7 (sold)