I'd say everything south of Bloor needs to be double width. Have you ever tried walking between College and Bloor during rush hour? I usually can't walk at my normal speed on this section because the sidewalks are so congested.
Id say if you are going to double width a part then you might as well double width the entire thing south of bloor as well. The reality is that with more and more development on yonge street there will be increased pedestrians so it is only a matter of time until it is needed anyways. If you start a project it might as well be consistent throughout.