Yes, it killed my schoolmate, but it took two weeks for him to die. And that was at contact distance. I don't think you can guarantee it will kill them fast, but the probability is high. I personally am interested in neutralizing the threat to me as fast as possible. This may result in the death of the interloper, but getting them to cease their attack is my objective. Unless they have take SWMBO hostage, I'll stick with buck for the first round, and slug (+ 3 00 Buck) for my second; I can eject the buck and use the slug in that case. I do have training in such situations, though it was with MP5SD3's.
The main issue with birdshot, and your salient point, is that it does not have penetration. There is a reason the FBI does it's penetration tests to 12"; this is the mean width of the human torso. (Some of us are thicker than that, some smaller.) Bird shot loses penetration fast as it travels forward, because it is not one .67 cal. mass of 1 1/8 oz., but 350 .095 dia. individual pellets whose cumulative penetration would equal a slug, however multiple round penetration is measured concurrently- each pellet has 3.04 ft/lbs. of energy at ten yards. That's 1064 ft. lbs., but again that's total energy, each projectile still has 3.04 ft. lbs. Compare that to a 1 oz. Foster slug at 2363 ft/lbs. Or 00 Buck at 1547 ft/lbs. That gives each of the 9 pellets just under 172 ft/lbs. , a medium pistol round equivalent each. This is why it is often said firing a round of 00 Buck is like firing a burst from an SMG, but instantaneously. Each of the 9 projectiles can kill, depending where it hits, not true for the birshot round, unless your invader is a pheasant.
The cumulative effect of 305 lightly penetrating pellets will cause significant surface damage, and some subcutaneous damage. Your idea of shooting the neck makes sense, as this target may kill, and will most likely cause your attacker to retreat if it does not. This is assuming he is not on certain drugs, or drunk. Then all bets are off, and I want definitive fight stopping power. The old Ranger/SF maxim always applies; "If you're in a fair fight, your tactics suck." I want every advantage I can get.
Make mine buck & slug. I know Cooper's Fourth Rule, and follow it.