I'm actually thinking about doing the same thing you are, it would be awesome to get it the day off instead of waiting for the shipping to finally roll out. I'm going to try eaton centre or fairview mall in toronto for sure also i believe apple pick up reservation should be back online that day too so if any luck find one in another store on the app and reserve it like that as well
Last time I did a line-up for the iphone (Iphone 6 plus) I got there at 3am and there was maybe 75 people ahead of me. I didn't get any chance at the 6plus. I could have gotten a 6 though. I opted against that at the time....
Moral of the story is I think you'll need to line-up the day before, not at 6am to even have a chance. Keep in mind also that every person ahead of you is able to purchase up to 2 devices. They go pretty fast. I'm also assuming they will have some available for walk-in's. They may very well not at all.