What an interesting question for the wee small hours.
I can only give you my own view.
I assume the '2 sub-levels of progression per year' is only meant to be a rough guide, although I'd be really interested to know what experienced teachers think about this.
My experience of appeals is that one very rarely got a complete SATs history for any child. The focus tended to be on year 6, with perhaps a snapshot or two from earlier on. If I wanted to know about KS1, I usually had to ask - and a lot of parents either couldn't remember, or were trying a bit too hard to come up with the 'right' answer!
Would I have accepted a 5b prediction as evidence of GS ability? I would have regarded it as a starting point
I would have been interested in the progression from year 5 to year 6, and what it was based on (e.g. optional SATs as well as ongoing teacher assessment?), but quite often such information wasn't available.
Ideally I would have liked the evidence to be as wide-ranging as possible.http://www.elevenplusexams.co.uk/11plus ... rs.php#b11
I accept that if the result were very marginal (e.g. a point), it should be easier to persuade most panels, but I suspect they'd still be looking for something more than a SATs prediction.