A called shot coup de grace is as redundant as setting a burning house on fire.
Coup de Grace - your enemy is helpless. You have all the time in the world to line up a shot (or ready a strike) to just the right area to kill the person. The other person is helpess somehow, be they unconscious or tied up or having their back turned to you thinking you are friend.
Called Shot (assuming people allow these in their games) - instead of just aiming at a person or creature, declaring an attack to aim for a specific body part or appendage. This usually involves a penalty to attack rolls to model the fact that you are voluntarily choosing NOT to aim at the biggest area of the target. This is a combat maneuver, and can be made against an enemy aware of your presence, able to dodge, or otherwise not helpless.
The point is that you cannot coup de grace IN COMBAT. One's foe must be subdued or left unaware of threat until the deed is done. Aiming for a target's head is NOT a called shot coup de grace. It's simply a called shot to the noggin. Same for groin shots.
Just remember this simple rule: "You can't Coup-de-Grace until he's down on his face!"
Well, you get the point.