I don't think I've ever shown up at YTZ much earlier than 35 minutes before departure and have never even been close to missing the flight. I have a Nexus card but have never needed to use it at YTZ since there never seems to be much of a queue for security. However, that's probably a function of the time of day I'm usually flying out of there (either first thing in the morning or mid-day - i.e. not during the evening rush) so YMMV. And this is for domestic flights, so I'm not sure whether security etc. is slower for flights to the US (though I would imagine not since I don't think YTZ have pre-clearance yet).

35 minutes for a US departure at 5.30 PM may be a bit of a risk, though not an outrageous one. I'd be more worried about being delayed by road traffic than anything else - "assuming" that you will be stepping out of your taxi at a given time during rush hour anywhere in central Toronto is not generally a safe assumption.