1) FMJ ammo can kill and if hit dead on it can stop an opponent right in their tracks.
2) When you are moving, shooting, and being shot at, you will fine perfect placement hard to accomplish.
Therefore you should use the best ammo you can get and practice (I mean combat shooting, not target practice) as much as you can.
If you do the above and are lucky then maybe you will shoot well even while your hair is standing strait up!
And I will say that if I had a choice of FMJ ammo and lots of practice or fancy JHPs and little practice, I'd go the FMJ route.
Skill and nerve is what matters, and nerve matters most.
“We can evade reality, but we cannot evade the consequences of evading reality” Ayn Rand