My DS(16) is in the top 0.1% academically and sitting GCSE maths early would, imo have benefited him greatly.
Primary school was either unable or unwilling to properly differentiate, despite numerous promises from the head every time I raised the issue. And my DS was one of those children who occasionally came home crying because of it. Boring lessons he could cope with, he would have even preferred sitting in the corner staring at the wall than doing the maths lessons that he was years beyond.
In Y6 I fought with the school for DS to be allowed to sit the level six exams (they didn't let anyone sit them as it was unfair on less able children). This wasn't because we particularly wanted DC to do the exam but because it might force the school to teach more challenging content (turns out it wasn't challenging).
DS loves maths it is his hobby, he does it for fun and covers all subject areas of it at home with no pressure from me at all.
Secondary is NO better. The school tried harder provided plenty of enrichment, UKMT, competitions, university visits, statistics, a mentor at Oxford Uni, trips etc etc but the problem was maths CLASSES. He still had to sit through 5 years of a classroom curriculum that he was years beyond. Differentiation didn't even scratch the sides. The new 9-1 system has not made a jot of difference either. He now hates maths classes and for someone who loves maths that is soul destroying. Good job the UK doesn't need gifted mathematicians.
Sitting his GCSEs at moment and not spent one second revising maths, no point, we are so glad maths classes are over.
So I am strongly in favour of sitting the exams early (for gifted children) because it would force the school to address the issue that he should not be sitting in the classroom doing a curriculum he is years beyond, it is more than counter productive. No extra teaching is needed, he could sit at the back of the room with a maths book. There would be no gap in maths between A-Level and Uni because he wouldn't stop studying it, why would he, he loves it. And that would be easily proved in the STEP/MAT entrance exams.