But if the playing field in general were more level then it would improve social mobility for all children from less well off families - including those who would be bright enough to go to grammar school, but also including all the others.
HPFA sorry, not paying attention, was the post you mean on the other thread? It got long and I got distracted so I didn't read it all.
Odins but your middle-achieving children will do worse under your proposed system (or, as others have said, the grammar will be a failing grammar). Your town doesn't support two schools, and if there are two schools, one of them will be too small to be effective or will attract children from elsewhere - schools get closed all the time because they are too small full stop, and smaller schools find it hard to offer a range of subjects and to give their students a quality education so also get closed on the grounds of not being particularly good (has happened to a school near me).
It would be better to improve the early education and home stimulation of all the children in your town so that some of the ones with potential match your DC1's ability - meaning there's genunine competition at the comprehensive. Lots of comprehensives do good things with high ability DCs.