Pearson has been recommending 3 hours for international flights for a long time now. I have been playing along, and seeing huge lines I am not in. I have some status with the airline I fly most, so I skip a huge line, and Nexus, so I can use a much shorter security line for domestic and international flights, and that same Nexus means that for US flights it's quick too, and I seem to end up sitting at the gate for two hours.
Still, I see plenty of people who are red faced, flustered, angry, trying to get let into the front of lines because they're worried they will miss their plane, yelling at random strangers in front of them to hurry up, and generally having a really unpleasant hour. I have no desire to switch over to that team by waiting a little longer until I get to the airport. I understand you have a sort of "elbow" around public transit, but I would still recommend you follow the 3 hour suggestion for an international flight. Weird long delays do spring up (sorry, we have half the security staff we usually do) and while a cab is expensive, it's cheaper than a new plane ticket.
Note: you don't mention whether your domestic connection is in your target country, or in Canada. If your first flight is to another Canadian airport you don't need to be there 3 hours before. Air Canada, for example, suggests 2 hours for domestic flights.
FYI my flights this year were April, May, and August in Terminal 1 (all international) and September (cross border) in Terminal 3.