I live in Toronto and the traffic there is terrible. YYZ is not a bad airport. Since you're flying Air Canada I would buy a pass to their lounge. There are other lounges that aren't bad either. Plus, you get food and drink, at a price similar to a dinner and a drink in the restaurants. There are other things you may want to see nearer the airport that would be less of a risk traffic wise as well.
I travel a lot through YYZ and I think it is not a bad airport for a long layover.