I have done this many times. Air Canada allows seamless travel via YYZ whereby your bags transfer automatically and you can bypass CDN Customs if you are immediately flying out of Canada in T1. DL and Westjet use T3 at YYZ and you must pick up your luggage and recheck. If you are doing this frequently you must get a Nexus card which is like GE and works in both directions and also counts for TSA Precheck at YYZ. Westjet still does not recognize DL elite status for waived luggage fees, upgrades or seat assignments. In my flying I have only gotten 50% MQMs on Westjet when the ticket is on Westjet stock but 100% MQMs when on 006 DL stock.