I get 50% extra time and rest breaks, I rarely use either. Yes, I do tend to get A*/As.
However, I'm visually impaired, several conditions you might not know - nystagmus which means my eyes are constantly moving and ocular albinism which makes me incredibly light sensitive. I'm also a hopeless insomniac. When I'm tired it takes me a lot longer to read because my eyes move more, and if it's too bright in the room that I'm in it takes longer as well. My eyes tire easily, sometimes to the point that I can't see clearly anymore.
Add on the fact that I have various anxiety related things and you get a tired, anxious mess sitting her exams, the only thing preventing a serious panic attack being the knowledge that I have my extra time to fall back on if it all goes tits up.
So, no. I don't think it's at all unreasonable. I don't need extra time because I'm stupid, I need it because it's possible that I'll read or write slower than everyone else.