Is Pre-Clearance really that big of a deal?
It makes transfer easier, yes... and it makes YTZ-LGA (or DCA in the future) possible.

for BOS, IAD, ORD, I don't see pre-clearance being a game changer. (And you no longer get to ride the mobile lounges at IAD! bummer)
If I were to connect at these airports, I might save more time by flying directly from YYZ unless you're going to some niche 2nd,3rd tier cities in the US.

I have Nexus, my experience may be a bit bias, if I were a frequent business traveller to the US, I think a Nexus card should be a prerequisite but I digress.

It works alright for Porter, not sure if AC can replicate their moderate success at YTZ.