The ones who have had illegal genetic enhancements would probably do what the Bashirs did and keep it a secret and try to live out their lives as best they can. We only know that Starfleet actively bans them, but the general stigma that seems to still hang around would keep most from wanting their secret to be known if they work or live with humans. They really shouldn't have to face this. For the few that are like Bashir should be able to pursue any career path they wish and to be treated with the same dignity as everyone else. This is meant to be our "enlightened" future after all.
The problem is that Bashir seems to be a rare case. DS9 gave the impression that the available methods and technology were more likely to create someone from the Jack Pack rather than a Bashir (I can't remember, but weren't the Jack Pack the least severe cases as well?). So I can understand why they would want to ban Humans getting unnecessary genetic enhancements if the chances are high that it is going to mess up the individual receiving the treatment rather than help them
As for working with the Ferengi or Romulans, they would have a worse time. Those outside the Federation don't seem too fond of ordinary humans, I'm sure they would like super-humans even less.
No, but Ferengi are awfully fond of latinum, and having a genetically engineered mind on their side would be very profitable. And the Romulans or most of the enemies of the Federation, you think they wouldn't take any advantage they can get and pay out handsomely for their services?
It's true we haven't seen any count of how many genetically engineered humans there really are out there. But there's also no reason to think that the five we see in DS9 are the only genetically engineered children out there. It's reasonable to think that there are some genetic kids on Earth, who are not mentally unstable, and are legally prevented from getting a job.