We are in Herts, so OOC, but v close to catchment for preferred school. Catchment Herts school is good school but struggling with sciences, where DS is strongest (and gifted and talented).
This is only relevant to you as a reason for wanting a particular school. The review/appeal is about whether your son is of grammar school ability in wider terms.
Head of current school has done appeals before and will strongly support DS on basis of his abilities and suitability, but as with all wd be new to review process.
Good, the stronger the support and the evidence, the better.
Am I right in thinking that if you have no 'extreme and overriding' extenuating circumstances, waiting and appealing on academic evidence might give a better chance than going for review now?
Not necessarily. If you have very strong academic evidence there should be no difference between the two. An appeal offers you the opportunity to present your case personally and to take school books in as evidence, whereas a review does not. A review, provided it is successful means that your application will be considered for grammar schools in the first round of allocations on 1st March.
And is the process more difficult/are we less likely to succeed with either review or appeal if we are OOC and do SATS not CATS?
The fact that you are OoC makes no difference whatsoever. Many Bucks children will not have CAT scores, only SATs. Some prep school children will have CATs, but only teacher-assessed SATs scores.
It is the overall strength of the evidence that matters.