When my wife and I travel to Sao Paulo on the 21:30 flight we like to arrive at T5 for about 10:00 - move between the CCR (we do outbound in First) and the First lounge every couple of hours and enjoy some plane spotting - so basically as early as you wish (just check lounge opening time (I think 5:00) to not be stuck waiting.

P.S. Nice one for the flights. I hope it will be a good trip and that you enjoy Toronto, we certainly loved it.