So sorry I didnt see this before
It sounds like sour grapes to me.
The academic standard of the HK students id very high and some of them struggle to integrate but it doesn't so any harm to the rest of the 6th form.
Average point score may be raised by them all doing GS and many taking Critical Thinking but even in the 4 'main' A2s (the standard, although some do take 5) the norm is A* or A.
If you and DC liked the school then go for it - the ethos at the open evenings is a pretty good indicator.
Opportunities for extra curricular and posts of responsibilty are varied and the high proportion of 'incomers' means there isnt an issue with CRGS/non CRGS divide.
My DS was CRGS through from year 7 (many years ago when there were fewer 'incomers' ) and he felt when he started year 12 that it was all very different and he had to re-adjust so its no easier for those boys in some ways.
If DC has a particular hobby or interest then that is a good way to integrate quickly and get involved with helping out further down the school.