I think that unfortunately it is hard to untangle the pros and cons of single sex education for girls from the fact that most girls' schools are indys or grammars.
Xenia, while I am sure you're right about the highest performers being single sex girls' schools, I strongly suspect that the boys also get the best results in single sex schools, simply because so many indies & grammars are single sex.
I tend to be pro, having attended a single sex school - I didn't experience any pushiness/academic hothousing - but that is probably because it was a not-very-academic comprehensive. Certainly compared to friends in mixed schools it was much easier to be good at maths/science without it being an issue - since everyone taking physics is by definition a girl, then you're not being strange by doing so!
Very sadly looking at the 'academically excellent' school that is one of dd's options when she goes to secondary it does seem that the 'girls don't do science' thing still holds, as a large majority of physics A level students and all of the electronics A level students are boys. So just for that reason I'd look hard at a girls school if we had one as an option (sadly we don't).
I suppose overall I'd choose a good girls' school over an equally good mixed school, but if the mixed school seemed substantially better then I'd go for that one.