I have seen a thing quite similar to your idea for Subaru's called a Phenolic Spacer and it gets its primary power increase by the extreme reduction of heat sink, it installs between the heads and the manifold though. But your idea of between the upper and lower should work similar. I study a lot on manifold design and it seems like your idea is worth the efforts. I'll buy one from you, but I think the material should NOT be metal

 

Almost two years later...