Justintoxicated , if you look at the docs for the Downboy it says that the emitter can not be grounded to the sink. It suggests using an extra layer of thermal adhesive to insulate it. But this would affect thermal transfer.
Maybe I'm being overly concerned here, but my experience with heavily overclocked CPUs has shown that a little too much thermal compound affects temperature. Contrary to popular belief, thermal compounds are not very good conductors of heat. They are just better than air and are designed to fill the voids where device to heatsink contact is not perfect.
For proper heat transfer you need as much device to heatsink contact as possible. When all of the thermal coupling is through compound (no direct contact) thermal coupling is quite poor and the device will run significantly hotter.
Thermal data on the Luxeons indicates a hot Lux puts out less light than a cool Lux, and will have a shorter life (lumen loss). I have a 1W Lux mod that is not very well cooled (small heat sink that gets too hot to touch), and I can see it dimming after a few minutes of continuous use. It seems to recover after it cools down (and I have run it for more than an hour continuously without failure so the cooling must at least be adequate), but I'm sure it will not last anywhere near its rated life.
Maybe I'm just being paranoid here, but my experience with the 1W seems to indicate that better cooling = brighter. Since the whole point of running more powerful 3W & 5W parts is more light, it seems to me that maximum cooling efficiency is a key factor.
Then again, maybe I'm just /ubbthreads/images/graemlins/crazy.gif
Aloha, iG