I just assumed that Bashir's problem was more academic. I mean kids in the 24th century do calculus (See TNG "When the Bough Breaks" or VOY Mosaic). So, I just thought that perhaps his impairment prevented him from being "normal" by prime universe 24th century standards but isn't so disabling in Mirror Universe society where academic excellence/intelligence isn't as valued.
That's a very good point. if there had been
serious problems, then genetic engineering was allowed.
"banning genetic engineering for anything except for repairing serious birth defects......"
However Memory Beta states
"Julian was born with serious learning disabilities, and at age six, he still had trouble telling the difference between a dog and a cat."
That's a long way below what you'd expect. Perhaps in the mirror universe he would have caught up a bit later?
If you can set that statement aside (where is it from?) as hyperbole, you could have Bashir's father, acknowledging his own failure to excel (in his view) in life, wanted more for his only son. He didn't want a son who was a moon-shuttle conductor and played tennis with friends at the weekend, he wanted a son that designed the next moon shuttle, while being a wimbledon champion (which as an englishman is very unlikely, no-matter how much genetic engineer went into him, and probably why he really gave the game up)
On a planet with a population of 5 billion, there will be 5,000 people there who find that they are the least advanced out of a set of them and a million other people. Doesn't mean they can't fight for their lives in a slave uprising though.