Business class does get priority security. The checkpoint is the same as the domestic nexus (security 3, on the upper level where the skytrain access is). Nexus goes down one side of the bridge, prio goes down the other. Nexus cardholders have their cards checked twice (at the start of the bridge, again at the end) before you get sent to the VT screening. Prio goes to the regular screening/ItD transfer screening side of the dividing wall.