There is also the danger of being a social outcast in a child's proper year, though, if they stay in it and are well ahead. I came from a country where acceleration/repeating years was more common, so the worry about secondary transfer didn't come up, but there was the possibility of moving me ahead at age 6. In the end it didn't happen as school and parents felt that socially it might not work. But actually, I often did socialise with the older year at activities etc, and I was always seen as a bit of a misfit in my proper year. I was the one other children came to when they needed something read, or help with something, or information or whatever. I wasn't disliked, I just didn't seem part of the class on equal terms - like an older sister in some ways. It was never ordinary peer relationships, and the chance for the ordinary give and take of friendships never quite worked. There were many other reasons that I didn't have lots of friends as a child - shy and geeky doesn't really help - but that contributed. I was physically as big and tall as the year above (being 2 weeks too young to have gone into that year in the first place), but wasn't very good at sports anyway whatever year I was with.
On the other hand, there are good things about being top of the class most of the time (although it didn't tend to teach me a work ethic, or how to try and fail at things). I also felt very out of place when even my own year were getting interested in 'teenage' things, because I wasn't. This was my own personality I guess, but if I'd been moved up, I might have felt even weirder (although maybe then I'd have at least had the excuse that I was a bit younger!).
I certainly learned to hide being clever as I got older; it's one thing to be fairly easily in top groups, and another to be noticeably different, and children (at least back then, and particularly in later primary and early secondary) were not very tolerant of someone who was seen as clever.
So I don't know. If she has to repeat year 6, then I think 'no'; but if not, it's worth considering her personality, the group that she'd be with, and how it would affect her more globally.