Have you tried flights via JFK or SFO?
I would personally pick the flight via LAX as I hate overnighting. Particularly if you are on AA646 which does not get to YYZ til 2330. If I had the time, I'd probably fly AA1552 (arr in YYZ at 1315), enjoy the afternoon in Toronto, sleep well, and then fly on to HKG the next day not exhausted.
Cheers,