6000K has nothing to do with brightness but rather the intensity. For safe driving, 6000K is the absolute max you should go for, anything higher makes it harder on the eyes. Our 121 has 6000Ks (fitted in Japan) and the Townace has 4300Ks (the kit which I installed), there's a significant difference with night time visibility, especially in the rain. If you want a proper functional HID system, go for 5000K (same as whats used in the Micro/SSangyong SUVs here in SL), and 4300K for the brightest possible simulated daylight.
As for heat issues, HIDs actually put out less heat, and the lenses are tolerable to touch even with the back of your hand. The visibility is great, filament lamps have nothing on HID. I drive long distances outstation on a regular, and the return trip is often in the evening, and the HIDs in my van have proved their worth over many times already. Not only do they increase my visibility, but in turn my vehicle becomes a lot more visible as well.