Yes, Xms headlights are poor to say the least. Ive often thought putting some candles behind the headlight glass might be more effective!
Driving in poor weather, at night, on a motorway can be interesting, you certainly wont fall asleep through lack on concentration thats for sure!
I think, since you have headlight washers, fitting a HID kit is perfectly legal. Many Xm owners have done that with great resutls. Or you can remove some of the headlight filters (the ones which go yellow, I have found that the strength of light can depend on how yellow the filter inside the headlight has gone) from the headlight which gives a much brighter light, though a slightly narrower headlight beam.
Im yet to experiment with upgrading the headlights, though might go for a HID kit for the estate.
Seems like your enjoying Xm motoring

You will quickly get used to the size, i dont even think of it as a big car, not compared to the width of a C5 anyway!
What do you think to the 2.1 auto combo?
Chris.