RE: "True Shooting Distance"
Gravity isn't creating more speed,atleast not in hunting distances.
Gravity doesn't know there is an angle there,it pulls in a straight line and that line only covers the distance from the base of the tree.
The same will hold true if you are shooting uphill.
There is a point when gravity plays a role with a downhill shot versus an uphill shot but it is at a much greater distance than taken while bowhunting.