The best available bins with current technology would be something like RW0H for the Luxeon I, UW0H for the Luxeon III, and XW0S for the Luxeon V. (I'm not certain about that last one, though.) Keep in mind that the Luxeon V is actually four Luxeon die in a 2x2 arrangement, and its forward voltage is about twice that of a Luxeon I or III.
It is not reasonable to expect an E voltage bin for a white LED, since these LEDs work by the production of blue light, which requires around 2.8 V as a bare minimum to get any light at all. This is basic physics and not an engineering problem that can be worked around. Future improvements in LED efficiency will come through designs that require less current to produce the same amount of light, or more light for the same current. The very low voltage bins correspond to emitters that produce warm, single colors like red and orange--not blue or white.
The very best conceivable bins don't necessarily exist in a real product yet, and even the best existing bins aren't widely used because they're rare and expensive. Sometimes you can be lucky; I recently found a RX0H Luxeon star in a cheap Brinkmann headlamp. As Lumileds' manufacturing technology improves, bins that were once very rare are now becoming more common. But getting something like VW0H bin Luxeon III, which is theoretically possible, will require a major advance in technology and not just an incremental improvement to the existing manufacturing.
