As a number of other have posted, our daughter (May birthday) has been something of a late developer and in the last 18 months has come on leaps and bounds. We are questioning whether it is worth detailing in the appeal letter the progress she has made in her SAT scores recently. A year ago her levels were 4C for reading, 3A for writing and 4C for maths compared with scores last month of 5C for reading, 4A for writing and 5C for maths? It is in the back our minds that if we mention 'late development' that this may lead the panel to question us on whether waiting for the 12+ would be more appropriate.
Late development as an argument is fine so long as the child is going to be where needed by the summer. The progress you outline above suggests to me she is on target for level 5s. I definitely would use the information.
Also our daughter's piano teacher and tennis coach have both, without any prompting, offered to write a reference and is something they have done in the past for parents going through the 11+ appeal process. The website seems to suggest that this sort of information is not considered by the appeals panel however is it worth putting it in if we do have it available?
Absolutely not - although if she's coming up to grade 5 in music, a letter confirming that would be fine!