It would be an advantage if it was intentional and both teams used the same ball. If you accept that under inflated balls travel further, the QB would have to practice with them in order to develop the right touch or feel to accurately target the receiver. Otherwise he would tend to over throw them. So it would hurt the opposing QB, causing overthrown passes. It would also imply it was no accident, that in order to be effective Brady would have had to practice. What would really make a difference is a quantification of the difference. I’m sure altitude, temperature, humidity, wind, all affect passing performance too. Would a QB even notice the difference due to pressure changes amid all the other influences?
“Would a QB even notice the difference due to pressure changes amid all the other influences?”
Oh, YES, we would notice. We can throw balls further which are under-inflated.