From what I remember, Bashir was enhanced because as a child he was retarded- while other kids were handling numbers, he was still struggling to tell 'dog' from 'cat' and 'tree' from house.
From Memory Alpha:
What they actually did to him was something called "Accelerated critical neural pathway formation", and by putting him in a new school with falsified med records they effectively covered it up and he turned into a genius.At age six, Bashir was small, not very bright, and a bit physically awkward for his age. In the first grade, while the other children were learning how to read and write and use a computer, Julian was still trying to tell a dog from a cat and a tree from a house. He never really could understand what was happening, but he knew that he wasn't doing as well as his classmates. There were several concepts they took for granted that he couldn't begin to master, but he didn't know why.
Of course, another part of the reason that such treatments were banned was because of the side effects- something Bashir himself was spared. It could be argued of course that the side effects were in part because of their clandestine nature. Had development been allowed to continue in the open they could well have eliminated the side effects entirely.