1 shot to the head and then they become injured.
Positives
More realistic and interesting RP because IRL you would not survive a head shot.
Players will value there life more and play situations differently and realistically in shootouts.
PD would no longer be able to abuse carbines and Armour.
But make SWAT have bullet proof helmets because you would not see a regular cop have a bullet proof helmet. (i am sure they are a thing in GTA Online)
Armour import price should then drop for criminal players as they would not be as effective.
Negatives
Desync could be a problem as you could be shooting there shoulder but it would register as a head shot.
If anyone else has any positives and negatives please leave them below.