A .40 caliber FMJ (especially a flat point style) can and will kill, no doubt. When compared to 9mm FMJ these would probably be significantly better performers. But for self defense, I would still prefer a proven hollow point design to any FMJ provided they feed well in my weapon. Buy a couple of boxes of Winchester Ranger 180 gr or Speer Gold Dot 180 gr and use that for defense. Use the other stuff for practice and plinking.
A .40 caliber is a fairly large bullet starting out. But the better HP's such as the loads I mentioned should get you very good expansion, perhaps as much as 0.60 - 0.70", even through clothing. That is a significantly larger wound track than could be made with a .40 caliber non-expanding bullet. And the sharp edges have a tendency to tear tissue as opposed to pushing it out of the way. Without a CNS hit, the next best means of stopping an attacker is with blood loss. The worse the wound, the more bleeding can occur and the faster he runs out of juice.
-Charging Handle