Two possibilities that need exploring:
I would want to know what the headteacher's recommendations were like as a whole. Scroll down to E9 (1) (d):http://www.elevenplusexams.co.uk/appeal ... laneous#e9
Just occasionally headteachers can be a bit too severe with their recommendations, meaning that a '2' is effectively the equivalent of the average head's '1', and a '3' the equivalent of a '2'!
The head may or may not be willing to give you the overall figures (but you could probably insist under the Freedom of Information Act, even though it might take time).
You would automatically get to see the figures set out in a 'grid', a week or so before an appeal - but I don't believe you would see them in the event of a review.
Secondly I would want to know the head's exact reasons for a '3' for academic potential. I can understand a '3' for attitude if output varies, but occasionally heads make the mistake of letting that affect their recommendation for academic potential. Is there plenty of evidence that your son is very bright? If so, it would beg the question - why a '3' for potential?
One advantage of an appeal would be that there's an opportunity to discuss matters such as the 'rounding down' and the headteacher's recommendation, and there's as much time available as is needed.
Review panel decisions, on the other hand, are taken quickly - see "Amount of time spent on each case"http://www.elevenplusexams.co.uk/appeal ... laneous#e9
and there is of course no two-way discussion, because parents are excluded!