I disagree with you on this. In another language with explicit public / private separation, the compiler can have access to the internal layout of a type (and thus optimise on it) without letting the developer mess around with it directly. I am assuming static compilation of course. Across a dynamic boundary, I would expect this compiler to behave like a normal C compiler and not use that layout.

In a header file, the information for the compiler and the user are the exact same which means you can't reduce your public interface without straight up hiding more of yourself.