Capoliton: It is a cornerstone of Thermal analysis. The rate of Heat conductance of a material is proportional to the delta of the temperatures between the hot and cold points. A stock 478 is not an ideal cool point in this case.

In actual fact what I suggest as a pure measure of conductance is to take the TIM's away from PC applications, and measure its effectiveness on between a heat loaded alumnium, and refrigerated copper. And push the evelope in to a delta of at least 100C at what ever wattage it takes to maintain this delta. Now. That would be interesting.