Most deep balls don't require crazy arm strength. I'm not sure why the deep ball is always included when looking at arm strength. Brady has always been able to throw the ball deep and his arm strength has never been amazing. Yes, if you are trying to throw it 80 yards, that's different, but that never happens. Deep throws are like 30 to 50 yards in the air and most NFL caliber QB's can throw it that far. The deep ball is more about timing, accuracy, placement, and loft.
The true measure of arm strength is in the intermediate game and getting the ball outside the numbers, as well as being able to make off-platform throws and make throws without being able to step into them.