John,
I think there is not much difference 'performance-wise' between solid copper or solid aluminum. Both have near enough the same thermal properties and hardness that I do not think it much matters. Obviously copper does have some advantage, else commercial head gaskets would not have used copper for years, but then they use steel now, so there is nothing sacred about copper. Making them from sheet aluminum has the advantage that the material is more readily available and cheaper than copper. In a wet environment, the aluminum might promote corrosion via a galvanitic reaction with the cast iron, but you do not leave your bike out in the rain, do you? I lightly greased the surfaces of the heads and barrels on mine, just in case it had a mind to 'stick.'
-Doug