I have two at Nonsuch (though both now in the upper end of the school) and I would agree with what others have said and not worry too much.
That said, I know what you mean about having irrational thoughts about schools and I have always gone with my gut feeling (heart over head etc.) I had a completely irrational obsession with one of the boys' grammars in Sutton but my son is now in year 7 at one of the others. I must admit that though he seems very happy and is storming ahead academically, I still find myself sometimes thinking "what if......" It's very strange as I have no rational grounds for thinking that and in fact there are things I've heard about the other school that suggest he is probably better suited to where he is. If you feel very strongly about it, then you can always add your daughter to the waiting list after allocations day and with a score of 247, you would have quite a high chance of being top of the OOC list.
Travel is a big issue though and in your case, if the journey is considerably further to Nonsuch than to Wallington, then you might want to ignore your gut and stay put in Wallington!
As an aside, the new head of Nonsuch seems to be making changes that I approve of
You will have seen the proposed changes to admissions (though I think the new PAN of 210 is huge!) and he is very much about educating the whole child. The school isn't perfect though and I have become increasingly disappointed with the sports coaching (as opposed to the opportunities) over the years. Music is fab and drama is much higher profile than it was when we first joined. Academic results last year were lower than expected and things have definitely been ramped up this year. There also seems to be a good crop of offers for Oxbridge/medicine etc this year (though you may well not be bothered about those things yet
Good luck with your deliberations