I was just at YYZ a few days ago and it was horrible! Worst that I have seen in 5 years -- took almost 2 hours to clear immigration due to long lines and far too few officers. Don't know how common this is, but it is a consideration. Also I've taken BA a dozen times through YYZ and every single flight was late (often less than an hour, but late none the less). Definitely not a good routing esp. if you start running into the last flight of day considerations on an award (where AA may not reroute you on AC).

YYZ has no interlining of baggage, so any checked bags must be picked up and cleared through customs. Which means you must also go through immigration. I think there is a sterile airside transfer int-to-int but only if you have no checked baggage and stay within the same terminal (T3 in your case would work). I've never tried this as I always want to go landside (if only for a few minutes).