I wrote to GTAA about metering and they said to contact the airline as they are responsible for metering. So I did write to AC, specifically about holding Nexus passengers for metering when we do not impact the regular immigration/customs line. See below for AC response.
Mods : I know this is Nexus related but specifically trying to get AC to change their policy on metering with Nexus as that's who I fly.I have taken the liberty of forwarding your comments to our Airport Operations Department for their information and internal review on your suggestion for Nexus passengers having the ability to deplane upon arrival before the other passengers.
Thank you for taking the time to contact our office. We look forward to the opportunity of welcoming you back on board again soon.