sonyaya,
do you think that would be at the expense of the most able students excelling
The thing is, my own experience both in terms of my own family and the schools that I know well, the limits that good 'true comprehensives' put on the attainment of the most able are very small, at least in terms of GCSE and A-level results.
The picture in terms of access to the most elite universities is more complicated - a mixture of lack of applications (the 'not for us' syndrome), lack of experience from the staff in terms of personal statements and university entrance exams, and lack of explicit coaching in e.g,. interview technique and those particular university entrance exams.
At the moment, the inequality of intakes 'masks' the true quality of schools (I have heard at least one headteacher splutter about the idiocy of an Ofsted / DfE -inspired 'partnership' with a local selective school, trying to help him to manage an intake with 45% SEN ..).
If instead of arguing about selectivity, we looked at where really good practice was - and looked at what works in schools with similar intakes - then that could significantly improve schools where there IS an issue with the performance of the most able, especially when all schools have similar proportions of such children. Coaching for, and sharing knowledge of, the entrance procedure for the most selective universities would also be an area to work on.