You mean the Crysis 1 KPA were more durable, right? One headshot amazingly didn't always kill them. It depended on the weapon and whether it was silenced. With a silenced pistol I think it sometimes took several close up head shots. It kind of bothers me that you don't seem to need headshots to quickly kill the human enemies in Crysis 2. I only go for headshots anyway though. Not that you need to conserve ammo in C2 as you did in C1. Especially if you wanted to keep the SCAR for as long as possible, which I took as another challenge in the game. It was actually quite possible to keep the SCAR right up until Core.
C1 depended a lot on the helmet (if they were wearing one). I didn't use the pistols for anything other than headshots on downed enemies (sleep dart from the SCAR--the sleep darts were uber against suited KPA) but a silenced KPA rifle would one-shot any nonsuited KPA unless they were wearing a helmet, and then it took two shots. One for the helmet, one for the kill.
I recall one-shotting helmeted enemies at times, and I think it was dependent on hitting their head below the protection the helmet offered.