Josh,
I can fit the HELLA xenon modules, in fact the best option would be the bi-xenon,but they are SUBSTANTIALLY more money. They would be easy to fit as they are incorporated (both high and low beam) into one module.
Comparing the halogen Hella modules I'm using now, to the xenon lights on my wifes VW Touareg, there honestly isn't a great amount of difference, especially considering the cost.
The downside to retrofitting xenon(HID) bulbs to the factory housing is that you still have the lousy reflector that is extremely inefficient, no matter how much light you put thru it.
The projector low beam HELLA and the freeform high beam HELLA are very efficient in their use of the light and are self contained and don't depend on the housing for directing the light.
I imagine that you could modify the back of the Hella modules for use of your conversion bulbs.
Bob