I'm confused about why formal honor codes are in this conversation. If they had any significant effect on plagiarism, we wouldn't be having this conversation. I don't know if my university had a strong honor code because it never once came up in conversation.
Strong (not formal) honor codes are in the conversation because my intended arguments for solutions to the plagiarism problems are predicated on there being value to a culture of honorable conduct in a university. So I was looking for real world examples.
Strong honor codes aren't the norm, so I don't think you could say we wouldn't be having this conversation if they had any significant effect on plagiarism.