I think you should make the distinction between amount of homework/work intensity for different years.
My own DD is coping fine in year 7, but then she's only half a term in, has no pressure and is still in the settling in phase as far as I'm concerned. There will be children who got next to no homework thoroughout their primary school years, so they're hardly going to be bombarded with homework as soon as they start!
My oldest chose not to go to a Grammar, however he cruised through Years 7 and 8 and is getting bombarded in Year 9 (as they start following GCSE courses a year early). My friends children in Years 9, 10 and 11 have a far greater work intensity than when they started. Of course they do!! However at this stage, I feel it's less about the amount of work (it is what it is) and more about how they are supported through their studies.
Incidently my youngest sounds like yours (without being in top sets for anything though) but for him we have accepted that the right school will choose him, rather than the other way round. He will cope wherever he makes the score and that will be the right place for him. We would be glad to have the choices you are aiming for!