The kettle grill is actually a pretty terrible design, from an efficiency point of view. The dome shape focuses heat back in towards the center of the grill, making a very small 'hot spot' where heating is even, and the black interior tends to limit the reflection of heat. This is true of barrel grills as well (like mine), but the effect is somewhat mitigated by the oblong shape. I say that as we prepare to buy a 22.5" Weber with the Kettle Pizza kit
because Mrs Tif is mad crazy about pizza and keeps giving me grief about not cooking it.
For direct-heat cooking, open air rules. You get better air circulation, which means more consistent burning of the fuel (for charcoal) and more even heating of the cooking grates (charcoal and gas).
The only time I would advise grilling with the lid actually on/down is if you have a rotisserie, which basically turns the grill into an oven. And I'm not entirely certain it's even advantageous then.
Incidentally, the reason most grills are painted black (especially on the inside) is because people hate cleaning them. Hides the guck better.