Biggest problem with transborder processing at YYZ is not at the check-in counters, but on the other side of the wall, waiting for the long line at immigration inspection to move forward. The only "fast track" is reserved for those connecting from same day flights. At 8 am it is likely to be crowded since the Saturday morning tourist rush will be on. Give yourself at least an hour prior to flight time to check-in, though you may not need it, then go to the MLL: very nice hideaway. With the computer situation, give yourself 2-hours even with J-class check-in.

So unless you plan to stay up all night with your friends, you won't have much time getting in at 8 and leaving at 8.

As for BA at JFK, same advice. It is chaos at curbside, less so when you get to the J and F check-in counters. From there it is an escalator ride up to the Club World Lounge where, if you are flying Club, you can have pre-flight dinner. That will pretty much decide how early you want to arrive. (You can also eat on the plane, they don't stop you from diing twice...) When your flight is called, you go down a back escalator where there is a dedicated security check to speed things along. Then chaos again at the boarding gate because the space is so confined.