I think you need to be clear about the grounds on which you might be appealing.
If your child didn't qualify in the test, I can see the point of as much evidence of academic ability as possible. However, the additional evidence would usually need to come from a recognised independent professional (such as a chartered educational psychologist) to carry any weight.
If your child qualified but the school is full, an appeal panel is unlikely to be interested in overall ability (unless score was one of the admission criteria). In this situation an appeal panel is likely to focus on your reasons for wanting a place. See:
http://www.elevenplusexams.co.uk/11plus ... ers.php#c2
Hope this helps.