In late August and September at night, that combo hopefully will get me close to zero* C, which in southern Ontario is fairly rare (and no elevation issues, i.e. Ontario is FLAT and near sea level.) If I sleep out in below freezing temps, that requires a rethink of all my gear anyway.
If I start getting into winter camping, I'll just go for a dedicated winter quilt system.
BTW where do you hike there? You're really lucky. I did a nice day-hike in the Blue Mountains to the Ruined Castle. Beautiful and interesting country, even without getting off the main paths.