You make some good points. That being said... I know from experience (not first hand) that being shot, anywhere on your body, is painful. Bullets actually burn. Its similar to holding a match to your skin. Its not like the movies when the killer keeps coming forward, taking multiple bullets.
Michael Brown wasn't a tough kid. He wasn't super human. I can bet he reacted just like any other human-being after having a bullet strike him. I get the speculation but at some point human behavior and the human anatomy come into play. Let's not make this kid out to suddenly be UNSTOPPABLE.
Officers are equipped with several different weapons that are design to "stop" an offender from doing harm. Stun Guns. Pepper Spray. Night Sticks. Officer Wilson MADE the decision to use deadly force and now the task of justifying the use of deadly force must happen. If Brown was THAT close to where he could physically harm the officer, it make sense to fire that many shots. Unless Brown was high on something or had a death wish, how can shooting someone 6 times (once to the head) be justified?
To my knowledge most Law Enforcement officers carry a Glock 19 or 22. Those weapons are DESIGNED to drop someone with just one shot. No matter the size of the individual. Point is... its a powerful weapon.
With that being said... I'm not SURE if the weapon used was one of the above but best believe, the police don't walk around with Pea-Shooters.