I have been looking into this for a long time and this really does seem like the easiest and best solution.
BMW do have a full retrofit kit available, Part Number is: 18.104.22.1684.195. But will not supply it to the UK market...
We have also tried a few of the 'non gen' HID kits and have found that when the HID xenon bulb is placed into the halogen housing, the filament on the xenon bulb is placed slightly different so the beam pattern is distorted and it doesnt look OEM BMW xenon so that kinda throw it out of the window anyway.
It is possible to buy the seperate parts from the xenon install kit and have the car re-coded at a BMW dealer with OEM Xenon headlights, but the cost for this conversion is around £1,500 depending on parts prices etc.
The possability of fitting OEM xenon headlight units and then powering them with a aftermarket HID xenon kit is the way forward for sure. There really is no difference once the BMW xenon headlight units are used...
Good post roofer.