Ironing
I can only speak from dd1's experience: Bog standard comprehensive.
Her year's been messed about with the new spec for maths. The school also did an experiment of splitting the boys and girls for maths which was pretty universally a disaster in the top sets. They'd done it before with success in the middle sets, but they tried it across the board for dd1's year. It was effectively a year lost as far as maths went for dd1. No one seemed to think it worked and they haven't repeated it.
What the school used to do was top set would do maths GCSE a year early, then do additional maths.
In her year they decided it was no point getting the earlier GCSE, so the top set did statistics GCSE in year 10 and were going to only do the new GCSE (why didn't they give it a new name!) in year 11.
However some of the pupils (and I suspect dd1 was one of them) told the teacher they really wanted to have a go at additional maths. So he set up an afterschool session. It was meant to be once a week, but was less often than once a fortnight. About 1/2 of the top set went and also about 1/4 of the second set.
They got a good smattering of As, and then right down to Us. Equally well they got (I think) 20/30 9s in maths from the top set and about 5-6 9s from the second set.
I don't think anyone got an A in additional who didn't get a 9 in GCSE, and I don't think any of the second set got an A in additional, although there was definitely at least one C, because one of dd1's friends got that.
For dd1's year, the right at the top mathematicians has been pretty consistent since year 7. There was dd1 and about 5 boys who would score consistently above the rest.
However a number of the 9s at the GCSE are students who have risen from where they started. Whether that's because they were badly taught before, or suddenly something clicked, or they worked really hard at it I don't know.
Dm was a maths tutor and sometimes she had someone who wasn't doing particularly well and she found there was something they'd just got a mental block about. When she got them through that they soared. There was one lad who'd been struggling in set 8/9 for years. She did 6 months of tutoring with him and when they did the end of year exams he rose into 3/9-and was at the top of that.
Additional is an unusual exam. I'm not sure how many take it, but I think it's in the small 1000s rather than more. I'm sure Noble will know!
I did it in 1992, and it was referred to as an O-level (called A/O) rather than a GCSE. It's now treated as an A/S level. Most people don't seem to have come across it, so I wouldn't worry about your ds not doing it.
The children doing additional were mostly those who want to continue with maths, especially those who are keen on further maths. Plus some who wanted to do science A-level but didn't want to do maths in the hope that will carry the maths content forward.
I think that dd1 reckoned that the new maths GCSE covered about 60% of the additional maths syllabus, so she might as well do it. However if she'd been struggling with the new Maths I would have been reluctant for her to do it as it would take away the focus. (although she tells me she only really did 2 days of work for it, between the main GCSEs and that, which was her last exam)