In a similar position we chose SMMA (and other parents have done the same) - so you're not mad for weighing it up.
There are the governors' place kids, plus likely same number again in siblings of those kids, who are often pretty smart too. And of course the usual number of bright sparks who live in catchment. The whole year does a maths and a VR test in June before starting, so the kids are set from day one for Science and for maths. A lot of the bright kids opt for Mandarin, so that ends up almost being like a top set too. The kids are set for other subjects later.
Leaving aside maths and English which all children do, the core academic subjects of biology/chemistry/physics + French/German/Mandarin all get 60%+ at Grade 7/8/9. We figured that if two-thirds of the kids in the classes are gunning for A's then that was enough of an 'A's vibe', in enough of the subjects we're likely to care about, that we didn't need to take it that step further by going to the grammar. (Likewise last year 13 kids got 7+ grade 9s at GCSEs - it just seemed to us a big enough cohort to be visible to lower years).
For us, it buys us an extra hour a day of family / activity time. I hope having a teen friendship group over this narrower area will allow for more / earlier social independence. For our child, we thought being part of a smaller cohort of bright kids would best help with keeping their confidence in their abilities high. Co-ed was our preference. At that time the info in the OFSTED reports was more appealing for SMMA.
Not particularly advocating here, just thought listing some of the factors that were relevant to us might be useful. We're very happy with our choice. The environment feels nurturing, lots of positive reinforcement and good comms from the school.
All the best for many happy school years ahead - whichever postcode you end up in!