I personaly suspect that it would never be spotted on an MOT test and if you actualy went out your way to maintain the lights it would still be safer than one hell of a lot of the cars out there!
exatly mark mot tester will not pick it up and would make a hell of a difference to candle power !!! how many cars are running around with only one headlight working !!! 
Not when they are dirty 
Washer systems are a legal requirement for a reason, I have hids on the 3.0 and it been dry for a few days, so not washed the windscreen.
Noticed my lights where not very good driving back one night (about the same as my halogens on the 2.2)
Quick wash.... and WOW... the difference!! Light output increased at least 60%
HID's are bloody usless without a washer system due to the nature of the type of light.
Spoken by somebody who has no clue about light clearly....
So, exectly what is the difference between incandescent lamp light and discharge lamp light? Can you actualy state what part of the spectrum is being affected by the crudd?
My view is this.....I cant see an insurance assessor even bothering about a well installed HID kit.
A well maintained HID light is going to be better than a poorly maintained incandescent lamp in ALL scenarios.
And no matter how you look at it.....if it stops more people driving round with front fogs on because the front controlled light output is better, then it can only be a good thing....