I have recently been doing some experimenting with LED lighting and large passive cooled heatsinks. One things I have found that is of note for people trying mods such as these is the large difference that SPRAYING the HS BLACK made to the temps. If you're using a HS that is designed for forced air cooling without good airflow, then you'll likely find the radiation limit of Ni plating hold you back. For example, with 8W of LED's, old 60W TDP Athlon HS with no fan fitted was resting at Circa 65C in still air. Adding a layer of Plasti Kote matt black dropped the temps by around 15 degrees C