Ben:
I've had a whole summer of playing in the high altitudes of Colroado. Here is what I have learned:
Height: The higher you hit the ball, the further it goes. My 8 iron, 145 goes 160 at 8000 feet, 155 at 6500 feet. I hit a high iron
My wife, who hits a low iron, gets about 5% more distance at 8000.
I would say 10% for a man at 8000, maybe 13 to 15% at 9500 at the Summit in Vail at the top of Cordillera