I had HID's on the VTS and I too miss them.

What I didnt think I would miss is the directional aspect of them. As you turn the steering wheel the projector units would turn left and right in the direction of travel. At first I thought nothing more of them as a gimmick, but on country lanes you soon realised they gave you that extra time to react to that sunken gulley cover or dodge the dead fox that you wouldn't have necessarily seen.
Even after two months of driving the A500 I still expect to see the headlight cast move as I turn the wheel

Full beam was good as the HID units "Flicked up" and an additional Main Beam bulb came on as well and were both directional. The only problem I found with the lamps is that on very dark undulating surfaces the HIDs would cast shadows across the surface as the beam was very flat.
HID ususally cant be added post production as the wiring set up is different. For a start they use the 12v into a ballast resistor to boost it up to something like 50,000v to ignite the bulbs. Of course depending on how much you want to throw at it anything is possible, but its usually easier to get a car with them already fitted.
There are HID kits avaliable which replace the standard bulbs but these are usually Chinese and very poorly made. They also break type approval regs regarding light output, lack of washers, no self levelling and the beam spread is all over the shop thanks to the reflectors fitted for normal bulbs.