I agree. If they took the school assessment, and 'progress made' into account, it would help level the playing field.
Way back in the mists of time (1971/2), in Surrey, everyone took the 11+. Some children passed. Others were near misses, who were then given interviews by teachers from the local grammars - who also looked at their school work (and maybe reports from the school). The best of these were also offered places. That worked well and was fair. The only child I knew who was heavily tutored was asked to leave the school at the end of the 2nd year, as he just wasn't up to speed with the rest of the cohort.
The school's are well aware of their most able students, but on an academic front only. The able students, who are able in other area's such a music, sport, chess, tennis, art etc shouldn't be overlooked either.
That's why schools become specialist schools, including some of the grammars. But what if you're brilliant at music, yet your grammar is a sports specialist school?