You might want to double check the pricing; I've seen DL approx 30% cheaper than AC for SYD-east coast. Maybe ex-DTW prices are higher (often that way for captive hubs); you might want to check prices ex-YYZ on DL (via ATL .... you can get 777ER ATL-LAX-SYD much the same as AC's YYZ-YVR-SYD same-plane service), or ex-BUF/CLE/etc. Even QF was pricing below AC for some of the dates that I checked (and QF normally is the highest, having a fine product which commands a premium over DL/AC/UA).