I would also say that 8s / 9s at the end of EYFS do not necessarily translate into 'higher ability in numeracy or literacy in Year 1'. The skills needed are rather different and it can give a false picture to equate higher scores at EYFS with higher NC levels. Also, as other posters have said, you do not know the total ability profile of the year, just that of your own child....
If your child is as able as you have described, then she should be capable of showing that ability in her recordings of what she does - written work, calculations, jottings etc. Those do not depend on whether she is introverted or extravert, so if she is producing good quality written work, then her ability will be recognised and the groups re-organised.
It may be that her lack of social confidence is not allowing her to show what she can do in a school context, and so she has been grouped with e.g. a group that frequently gets TA support to encourage her more than she would in a more independent group. I have sometimes placed a slightly more able but not confident child in a group whose work she can manage easily beore giving her the next group's work as an extension, as some children need to get their confidence up through doing something they find straightforward before they are willing to tackle challenging work.
Do you have examples of excellent unaided work that she produces at home e.g. writing that you could take into school to discuss with her teacher? Teachers will always take such things into account as they give a much more rounded picture of a child ... but get rather grumpy if the 'fantastic' work shown is demonstrably done with mum at the child's elbow giving her spellings or ideas or even (boy in my class last year) writing the whole thing out for child to copy!
Also, I would expect groupings in Year 1 at this point in the year to be very fluid, possibly even mixed ability or based more on who works well with who rather than 'pure' ability. It is early on in the school year, the step into Year 1 and more formal learning is one which involves adjustment, and you might find that steaming into school with a 'she's in the bottom group' wrong-to-be-righted agenda is in fact based on a total misconception of how the groups are working.