If you check head temperatures in multiple locations you might find one head is vastly different than the other. That might be a clue to remove that head first to check for a gasket issue.
and yes, you can tell yourself that it is the restrictor plate and the thermostat. It may well be the thermostat, but as I mentioned earlier, it is rare for the restrictor plate to have been removed.
answer your thermostat questions before you consider actually removing either head
Larry