My DS started at SMMA this September. It's way too early for us to give anyone chapter and verse on it, other than the fact that he loves it so far. He's not in the 'high-achieving' bracket, just fairly bright, albeit scatty. 'Lazy middle attainer' might be a good description for him too. The level of homework was a bit of a shock, but I have no idea how that compares with HG. He says 'they work you hard' but seems fairly happy with that, mostly...
A couple of points in response to comments:
"(T)he catchment is teeny now because of the faith places, the primary school places and now the governors places."
The governors places (aka 'language aptitude') are not based on catchment at all, anyone can apply. That's how DS got his place; we are way out of catchment.
"(I) get the impression at SMM you get established groups arriving from faith primaries whereas at Highbury Grove there's more of a mix up"
I'm not sure that's 100% true, though I'm sure there's truth in it. At DS's primary the staff assumed quite openly (quite annoyingly; it's as if any other schools simply didn't exist!) they would all go to Highbury Grove, so there are certainly blocs of kids arriving there in a piece also. SMMA has this of course, but DS seems to think there's a good mix.
Still getting a feel for the place (secondary is very arms-length for parents so it's a slow process) but you can DM me if you want to ask anything. I signed up for the weekly school newsletter well before DS started to get familiar with the place & I'd recommend that as a starting point.