What great improvement?? He ran sub-20 as a kid (what, high schooler or something you'd say? 16? Same age as Mary Cain?) once he grew to his full height. After regaining the co-ordination loss that comes from switching to a full adult form and filling out he then improved say 0.7 over 200m, 0.35 over 100m. It is likely he was just sub-10 or could have ran it in the 100 for a long time before 2008. Please don't harp some 10.03 time at me in response, which was his first competitive 100m race.
You could say that others like Gay have had bigger improvements than Bolt, especially when they were precocious, but not as precocious as to have WRs from age 15 onwards.
Would you call LeBron James, Michael Jordan out for a similar 'improvement' to be the world's best? I don't know about Basketball but I imagine that they were always great and stood out at every age group? I've heard that about LJ I think to be fair.