I spent about 30 to 45 minutes testing the headshot fpi (appearwithheadshot.fpi) using headshotdamage. Hey, I'm retired, I can spend my time now anyway I want.
If you set it to "25" it will take six shots to bring em down. Set at "50" it will take 3 shots. Set at "75" 2 shots. Even set at "100" it will take 2 shots. Set at "125 will take 1 shot. If you want just a 1 shot take down, then leave it alone (don't add it to the script and use the default).
Also, the headshot will work with neck to maybe midchest but not a direct hit on the shoulder. I imagine it has something to do with the limb configuration.
I used Thug (pistol) for the tests and used the sniper rifle with scope so I could see exactly where I was shooting. Whether this agrees with Lee's tests I wouldn't know.
Personally, I would use headshots sparingly as why would you want the player to win so handily. Aren't games supposed to be a challenge?
However, I can see it being useful to add variety. Like sneaking around a guard without him seeing you and then pow letting him have it with a scoped rifle. This was talked about in another thread and I can't remember where that was.