One of the ideas behind LED's is the long term savings. It would be nice to see a side by side of a four year operating LED and a new one of the same make and model. The reason I posted this comment here is because of the talks I have had with other HGL users and how there is a common belief of the quick depreciation of performance.
That's a long side by side lol. Maybe just check output with a light meter from an old one an a new one. Sounds like a good idea though to see how much the light is diminished.