What Viperoni said,
and,
Think about this. The shim touches a warm heatsink on one side, and ceramic substrate(cool substrate) on the other. THe shim gets "warmed" up by the heatsink, and may warm up the Substrate(probably very little, but it would negate any potential cooling gain, as it would be small if there was any).
Not to mention AMD recommends against shims.
Mike