The choice of metal for a cylinder head does not effectively lower the compression ratio in any way shape or form.
Put both, otherwise identical heads on an engine and do a rotating compression check in both will have the same psi. Aluminum has the effect of transferring hate quicker. And that is it. It does not lower the compression effectively ratio whatsoever.
This test has been done several times over. It’s in print several times over. This has been proven over and over again.