On a side note to this, the headlights themselves can degrade over time too.
I recently bought a set of Volvo xenon headlights to strip for the projectors to retro fit into another set of lights. If you didn't know, the bulb sits in a reflective chamber (much like a reflector headlight) with the projector lens in front. In the case of the Volvo, where the bulb sat closest to the reflector, some of the reflective coating had burned away. This would obviously have a detrimental affect on the light output.
With the Mk7 getting on a bit now, it's likely that something similar may be going on. The trouble is, there is no way of knowing without striping the headlight down.