I'm not sure why, but as a kid in the 70's and 80's I was quite certain the stormtroopers were all clones. Maybe it's mentioned in one of the Han Solo books or a Lando book or what have you. I know I didn't arrive at that conclusion on my own. I always thought that the soldiers in drab uniforms were real human recruits and the troopers were clones. [In one of the books, Han Solo's backstory involved him training to be an officer and losing his rank when he saved Chewbacca.]

Also, didn't Lucas say that Jango hitting his head was meant to suggest that his clumsiness was passed-down to the stormtrooper in the original Star Wars? That sort of ends the debate.

As for why the stormtroopers can't shoot worth crap and don't speak with Kiwi accents -- well, assuming Lucas doesn't re-dub every piece of Stormtrooper dialogue for the next special edition (and I bet he wants to) -- I would sort of accept the idea that, like the technology of prequel universe vs. the original films, the clones have dulled under the rule of the empire. The empire doesn't "grow them like they used to," and that's why the troopers have bad aim. The clones could have lost their accents over the years because they're not being grown by the same company by the time of the original Star Wars (and why would an accent be genetic anyhow?).