When I flew that flight ( a lot) I arrived around 7 and walked right to INS and the rest was pretty fast. YYZ has a million USA flights at 6 to 7 a.m. so there is a throng from 415 am onwards, then it is quite quiet until later on and there is no sense arriving as part of the group rushing for 7 am flights, so getting there after they should all have boarded has always worked for me. Arriving at 6 you will get in others' way and waste your own time.

There is an AC and it is nice enough to wait in as well.