How well accepted is the 'STAR' test for reading age?
If you mean 'How well accepted by an appeal panel?', I doubt that they would usually have a view about which reading test is used.
They ought to be cautious about any
single test result (I occasionally heard appeals where the child had taken two different reading tests on the same day, delivering widely varying results - 2 to 3 years’ difference!), but even so, I don't think anyone is going to quibble over 15 years and one month! It should be fine.
How well accepted is BAS II?
For anyone familiar with EP reports (and there's likely to be at least someone on an appeal panel who is), BAS II is a standard, well-respected test.
To me, his results look great (he's 99 percentile for 7 of 8 tests, 95 percentile for the 8th) but how impressed will the KE appeals panel be by them?
The results are excellent, but I'm afraid there's no telling how any individual appeal panel will react to a particular bit of evidence.
Is there any specific research or additional info that I'd be wise to research before the hearing? (assuming it gets that far)
I don't think so - assuming the EP has provided some explanatory notes within the report, and discussed the findings with you so that you broadly understand the main points.