This question came from a topic discussed earlier, where people were wondering whether a fit 40 year old guy could run a 27 second 200 without specific running training. The consensus was no:
http://www.letsrun.com/forum/flat_read.php?thread=3175953
But what happens if we're talking about an average guy fresh out of high school who was trained optimally for the sprints? Would he be able to break 27? I'm going to say yes, since many elite marathoners can break 27. If someone with very few fast-twitch muscles can do it without spending much time on sprint training, then the Average Joe with 50% fast-twitch muscles should be able to do it after a lot of speedwork.
Anyone else want to share their thoughts?