I have a friend who got diagnosed with dyslexia so got extra time in exams but as he once said himself 'having extra time doesn't help if you can't actually read the question.' Plus he always finished way before everyone else anyway and you would see him sitting there with the paper closed when we all had to get up and leave.
I had another friend who got diagnosed as a 'slow writer' which seemed to basically mean she could hand her coursework in 6 months after everyone else. I don't think it's particularly fair that slow writers get more time however my brother appears to be a slow writer so we're trying our hardest to get him extra time, just because it's available and he'd actually be the one at a disadvantage if he didn't get it.