It was in the low 50's (about 11C) outside on game day.
The balls were found at 10.5 psi, and the minimum Regulation pressure was 12.5 psi.
So, 84% of regulation pressure means (since pressure is proportional to temp, all other things being equal) that the balls would've had to be inflated in a 338K environment. Which is 150F.
I suppose the Pats could've inflated the balls in a sauna, but it's rather unlikely that the Refs would've failed to notice that the balls were hot enough to burn them when they checked the temps before the game.
In other words, no, the Pats cheated. Did their cheating matter in the end? Nope, the Colts sucked so much that day that the Pats could've played fair and won.
Alas, playing fair isn't something they're all that familiar with.