Are files with 755 permission are a security threat for the server?
No they are not due the fact that when you sign up with us your public_html folder is created with 750 permissions. These permissions allow all actions on the files for the user and only reading and executing permissions on the group which in most cases is the nobody group. For the other type of access which is for the world the permissions are set to 0 which means that there are no permissions for the world group.
Note that these settings are for the public_html folder and all files located under that folder no matter what permissions you set to them will be secured due the above provided explanation.