I think it officially kicked off at 4oF, and all beer was poured from bottles as the tap lines were frozen. You had to poke a hole through the head of your beer to take a sip.
And you're right, assuming the volume of the footballs didn't change much if they started the game at 12.5 PSI they would have dropped to 9.6 or so in that temperature range. I still think Vinatieri was kicking a rock around that night.