I cannot claim to know the difficulties of anybody I know to have extra time, at least not to a sufficient standard to deduce if it puts them at an advantage or not, because I am not them, and I have limited understanding of the sorts of issues that lead to extra time. In general, I like to give people the benefit of the doubt. So if someone seems quick-witted and fast at working in general, and gets insanely good marks, but they also get extra time, I'm not going to get all jealous and think "they're cheating the system!" cos frankly I have no idea if they are cheating the system and the odds are surely that they aren't anyway.
However I often wonder if I could have got extra time as I've suspected I have some degree of Asperger's for a while and in the early parts of exams I set off like a snail it seems, being very slow to understand and answer the questions (obviously this can have more of an effect in the shorter exam), but then the longer it goes I get more into it. Maybe that's just normal, I dunno. Also my handwriting is right on the precipice of being illegible and I've been told multiple times it could royally screw me (not exactly in those terms) in exams. It hasn't yet, but this term I think my writing has been particularly bad, so we'll see what my results say..