I think that thermal grease thing was started by the manufacturer of Sil-Pads, the competitor to mica. Mica is actually a better thermal conducting material than the sil-pad. The only advantage the grease offers is a theoretical advantage of more of the mica getting in contact with more of the heat sink. But - if the mica surfaces that were in contact already now have a less conductive grease between them, is it really a net gain? You would have to get out the infrared thermometer and do some measurements.
Ironically the best thermal conductor is water. The darned stuff has a bad habit of running all over the place though. And boiling off if it gets really hot.
Speaking of water in contact with metal, have you guys seen this?