Thanks pmichelazzo. Yes, I was a bit worried about the time taken to go through immigration at GRU. But actually, I need to catch the flight at 16:45 from GRU to POA. So if I can check in my luggage early at GRU at the LATAM desk, I think I can go over to the LATAM lounge at terminal 3 and wait there (for hours) for my flight to POA. But this is only possible there is early bag drop off at GRU.