If you take a gun that has 25 damage per shot, a 1.34x multiplier turns that into 33.5 damage. That reduces the amount of bullets you need from 4 to 3, but only if all shots are headshots.
If you up that value to 1.75x, it does 43.75, it's still a minimum of 3 shots to kill, but now you need only 2 headshots, while one can be a body shot. At a 2x multiplier, you'd go to a minimum of 2 bullets, if both shots are headshots, or it takes 2 body shots and 1 headshot.
What this tells you is that the lower the damage of the weapon, the higher the headshot multiplier must be to even make a proper difference. On many weapons in the game, increasing the multiplier to 1.75x wouldn't drastically lower the TTK.
On the contrary, many of the higher damage guns like the AK4D, do slightly more than the advertised 33 damage. They do around 33.4 or 33.5 damage. What this means is that they need only 3 body shots, but as soon as you have 1.5x multiplier, it goes down to a 2 shot kill if both are headshots. In comparison high damage guns benefit massively from headshot multiplier changes.
