Fair enough if the comp is a better fit and that's what you decide in the end, but I do wonder a bit about some of the basis of your decision.
Clearly the ethnic and social mix is important to you - fair enough. However, as a previous poster said, the 91% English not first language and 60% FSM might reflect your community,but doesn't reflect the country or other communities she might live in later. Is there any reason to think she will always live where you live now - is there a chance that by only exposing her to one type of community, it becomes harder to later to break out and become part of others, which might be benefitical in all kinds of ways at later points?
Clearly your other DD have done well and are doing well. However, it's also worth all children knowing there is more than their narrow and limited experience, and I donthinknsome parents either knowingly or unknowingly limit their children's experience.
This just reminds me a little of a conversation I had with a fellow mum - she had a choice of 2 schools for secondary - a really good one which was Ofsted Good and doing really well - very big school with lots of clubs, good GCSE results and wide ranging provision, and the other school which was struggling - far smaller so struggling for funding and applications,with much worse results and limited clubs and low morale amongst staff after poor ofsted. The Mum was choosing the latter - why - because she had gone to that school and her older 2 had too - her words 'it was good enough for us and it will be good enough for DD' - there was a real sense of fear about this other unknown school which might contain children and experiences she didn't know about. This woman had a life of struggle in all kinds of areas and it felt to me like she wanted the same for her DD and that the idea of aspiration or something better just made her personally feel threatened.
Now I know OP that your situation isn't like this, but some of your comments reminded me of this. I wonder if the GS is as bad as you make out - you make it sound terrible in terms of extra curricular, and that sounds very unusual to me. You also gloss over the academic success, and seem sure your DD couldn't make friends there or be happy - and that 'those people' are just not her type and she wouldn't fit in - but is that really true, or is it your own prejudices limiting her?
I'm not saying the GS is the right option. We could equally be talking about an alternative Comp couldn't we really. And I'm not saying the school all your kids go to and which you clearly rate isn't good either, but I'm asking if you should look again at some of the basis of your preferences and question them. You may do that and still reach the same decision. You might think it out of order for raising these questions and I don't know your area or circumstances, which is all true, but you have come on here to ask, and some of the things you mentioned just made me wonder if you are really open to all the alternatives.
You say your DD has decided against GS now. Well, she's changed her mind in a couple of days and this really might not be the last change of mind by a long way. Wouldn't the best thing be to look at all the alternatives with her anyway, encouraging her to be open minded andntryingnto be so yourself too,rather than pouncing on today's view of it, because her view today fits with what you want. Just some thoughts.