Can you expand on that?
In a nutshell when a car lamp is designed the standards it is designed to vary depending on class of car, size, height of the light source and the type of light source.
Dipped, position and high beam are measured to certain standards over 25m, no matter what the market Europe (UK), US and China (these are the main ones which most countries reference for their lighting standards).
In this case the light source:
A tungsten lamp (Halogen bulb more commonly known) as we know has a filament which burns over time. The reflector of the lamp has been designed around that type of light source at a specific orientation and position. Changing the light source changes for focal point (distance from light source and reflector) which will change the performance of the light over 25m. Even having the correct light source and rotating the bulb within the reflector will greatly effect the light pattern and is very likely to make a car lamp illegal.
So when a lamp is granted a approvals (for all functions and environmental performance) it is to that light source type. A change would make the lamp design void.
There are many classes of luminance standards as previously mentioned but the main one is the standard (Halogen) and High Intensity Discharge, which includes zenon and matrix beams (LED lighting). Though the light pattern remains the same for two different light sources for the same market the light output varies. The area of particular interest is light above the cut-off (light above the light pattern) as this is the 'glare' we refer to when driving.
If we take HV point 0,0 (point where the straight line kicks up to the angle for European standards) can not be above 0.7 lux for a halogen bulb, but for HID can go up to 1.0 lux. I know this doesn't make sense why one can be higher then the other but it just is. The MOT man can not measure accurately lux and concentrate mostly beam alignment and light pattern shape that is why you can pass your MOT with this mod. From my understanding they can't measure specific test points.
All HID need washer system and levelling is only needed depending of the height of lamp from the road. The reason for this is because a dirty lens changes the refraction angle causing glare (possibly) and as the light source is much brighter there is more light to refract around (this may be the reason why halogen bulbs don't need washers but the light above the light cut-off is only 0.7). When testing lamp washers calibrated dirt is used and optical performance is then measured.
Then there is the introduction of the ballast. From a light performance point of view there are pros and cons.
The pros are the heat generated from the ballast can bring up the dew point of moisture within the lamp (no lamp is water tight) thus reducing the risk condensation building while the extra heat also aids burn off time. (A lamp should be I believe 70% or more clear after 30 minutes of being turned on). This is why on some headlamps (particular long lamps) have the ballast put under the lamp housing and towards the front (typically the coldest point).
Con easier to spot you have done the mod. The extra heat (though not likely) can effect the life and the colour of a LED light source if the lamp isn't designed for a LED light source. As LED's get hotter they get brighter and colour changes. This is why when LED lamp functions are tested with a correction factor for brightness and colour, again the MOT can not test for this as far as I am aware. After market LED retro fits don't come with a heat-sink to prolong and standardise their performance (well the ones I have seen).
Sorry if I went on a bit