I have used both HID and high performance halogen lamps for years now, so I have seen the difference in light output as between both systems. I have yet to find an HID headlight replacement that offers HID for both high and low beams (bi-xenon lamps), and were actually manufactured for highway use. The only bi-xenon lamps I have seen were OEM on newer Audi's. They work very nicely, but retrofitting such an engineered system to another vehicle would be painful at best, and very expensive.
Unable to find an HID retrofit system for 7" round lamps that performs well, I have settled for HID auxiliary lights and halogen headlights. I can readily find what I need by going this way, and you can get excellent performance from halogen.
However, if you really want HID-type performance from 7" round lamps, or any other light that uses H-4 bulbs, follow the link provided in the first post under this thread to ARB USA's bulb selection. Under performance bulbs (not the ones that supposedly give light equal to high watt bulbs, but those which are actually rated as high watt bulbs) you will find H-4's rated at 100/170 watts. I have used these in the Aussie Outback and on high beam, I could not readily distinguish between the halogen lamp light and the HID 6" drivers when both were on at the same time. The downside to using this high a watt rating is heat and electrical consumption. You will need an alternator that can support the watts and you will need to rewire your headlight circuit to minimum 10ga wire running through relays direct from the battery. You will probably also need after market bulb sockets, as H-4's at 170 watts make a lot of heat. I melted both sockets the first time around using stock sockets.
The point is that if you want both high and low beams built into a single H-4 style headlight, you will likely need to stay with halogen. If you really want HID performance in anything other than low beam, choose a good brand of HID aux driving light.
During the 2007 Expedition Trophy, I looked at the HID's in the JK posted earlier in this thread. I have to say they were the best I have seen to date, but as noted, there was essentially no high beam to them.
As for the bi-xenon OEM lights and how they work, they use a moving vane to cover and uncover a single stationary light source which then shines through a lens system that has both high and low pattern light dispersion, depending on how much of the light is covered or not covered. 7" round lights, whether you have one with a reflector behind the light or in the lens, uses light from two different starting points, so simply putting an HID bulb into an H-4 light will not give you both high and low beams. One or the other, depending on where the bulb is located, but not both.
Using an H-4 conversion with a moving bulb could allow for high and low beams, but the moving bulb system would need to be high precision to return the light pea to the same location every time, and the reflectors would need to be designed for the kind of light source used. Since the reflectors in H-4 conversion headlights are designed for transverse halogen wire elements, not for linear HID arc lights, they don't as a rule provide correct light dispersion when used for HID light. Close, perhaps. Usable, maybe, depending on your perspective. But still not what they could be if built for the purpose. Daniel Stern has a detailed discussion on this topic.