Hi Paul,
You are on the right track adding any additional fields to the User record that you might need in a Email template or a workflow. However, you should not need to replicate these fields on the Lead, Account, and Contact as the fields from the Owning User are accessible from the Workflow and Email template. From an Email template, when you go to insert fields, you simply select fields from the User. From workflow, when you insert fields, you drop down the list of entities and select Owner, and all the fields on the Owner record will be available to you.
Does that make sense?