you'd
actually be dumping heat back into the substrate

Interesting theory, but I'm not sure I buy it, especially in the case of
a water-cooled system like mine. I can touch the perimeter of my waterblock
under full load, and it is essentially the water temperature. In other
words there is a significant temperature gradient between the outer and
inner areas of the waterblock surface, and I would imagine this gradient is
much steeper than for the average air-cooled sink, given the poor heat
removing capacity of air vs that of water.

The purpose of the copper shim is to increase contact from the substrate to
the outer area of the waterblock surface, which in this scenario is quite
likely to be cooler than the substrate, thus removing additional heat, not
adding it.

Also, in the ideal situation, with a more or less perfect heatsink that
instantly removes all heat without spreading it out over the heatsink
surface, it should be clear that more surface area equals more heat removal.
So the better the heatsink, the more a cpu copper shim should help. That's
my argument anyway.

No, I haven't tried a copper shim yet, tho I have bought one and have been
meaning to try it out, along with doing a simple waterblock mod and using
larger fans on my radiator.

rms