He will be academically pushed in any grammar school at any level, it's just whether it is gently encouraged or simply expected, obviously there are lovely teachers in both schools and harsh teachers in both too.
The silent room bit is very very important to get as much help with in the next two years as possible, grammars and uppers continually assess, it's no different from when we were at school, remember the monthly maths tests, chemistry, French...etc. jh has six assessment reports a year, based on testing and work, rgs has three a year. Both boys have some kind of test twice a week at least.
I know it seems like another mountain to climb amongst several you and he face, but focus and concentration on the task no matter what is happening in the room is hugely important. (Soneone please tell that to my dh when we are out in a pub together and there is a telly on in the room
Good luck on your journey, try the misbourne and risborough for uppers as well as the maybe chance of great Marlow if you are that side of town.