Will a spell like Fiery Cannonade kill Wayfaring Temple if Temple had more toughness than the damage before casting?
The Wayfaring Temple will die.

When Fiery Cannonade resolves, it deals 2 damage to each non-pirate creature, which in this case is 2 damage to Wayfaring Temple, 2 damage to creature A, 2 damage to creature B, and 2 damage to creature C. When state-based actions are checked, creatures A and B die because they have as much damage as toughness. Because state-based actions caused something to happen, they are checked again. This time, Wayfaring Temple is a 2/2 with 2 damage, so it dies as well.


  
  State-Based Actions
  
  
    704.3. Whenever a player would get priority (see rule 116, "Timing and Priority"), the game checks for any of the listed conditions for state-based actions, then performs all applicable state-based actions simultaneously as a single event. If any state-based actions are performed as a result of a check, the check is repeated
    
    
      704.5g If a creature has toughness greater than 0, and the total damage marked on it is greater than or equal to its toughness, that creature has been dealt lethal damage and is destroyed. Regeneration can replace this event.
    
  
  


Damage only wears off at the end of the turn; so if it survived, it would have 0 damage at the start of the next turn. But it still has that 2 damage for the rest of the current turn.