Just as a small hint to make things better: After extending the portfolio post type in the specular child theme I ‘ve seen, that some of the phrases are not present in the specular.pot language file. Such phrases as “Portfolio Item” or “Portfolio Entry” do not appear in the language file.

Would be nice to have all language phrases present in the *.pot file.

Furthermore it would be nice if the functions in /includes/register-post-types.php would be wrapped in an if condition, that checks if the function already exists. That would make overriding these functions in a child theme more easy. With the present solution you have to unregister a post type, before registering the same post type with different options. An if-condition could make it much more easier to extend post types in a child theme and it would safe time when developing a child theme.