Please don't worry, I have had two DDs go through gcse at Nonsuch - one still there in the sixth form and one just left. Neither of them are natural scientists/mathematicians so both were in the lower groups for those subjects.
It really doesn't matter a jot what group they are in unless you want your DD to do maths gcse in year 10 as only the top half of the year do this. Neither of mine did maths early and they both got A* in year 11. They both got A or A* for all of their sciences too.
The teaching in the lower groups is still good it's just at a slower pace which is more suited to the pupils in those groups. Remember though that a slower pace at Nonsuch is still going to be pretty speedy compared to a non selective school.
As for the triple science thing the school does do separate science gcses. It's actually compulsory at gcse except for a miniscule minority of girls who are allowed to do double because they would seriously struggle with triple science.
Nonsuch had an amazing set of results this year and there are lots of plans for the future of the school. It's a good place to be (but that's just my humble opinion