Haven't read the whole thread but from my personal experience, stick with one system from 12 to 18. I know that's not what you're asking but fuck it I'm going to tell me tale anyway 
I went from a very well-known and academically strong single sex to a well-known boys school who had just introduced girls in the 6th form and it was a massive failure for me, academically, socially and emotionally. There were 350 boys and 29 girls and it had this mad febrile atmosphere were the boys both lusted after and sneered at the girls, putting them down at every oppo, marking us out of 10 in the dining room etc etc. I was so stressed by it all I went from being an Oxbridge sure thing with a scholarship to the school, to it not even being clear if I'd get any As at A level.
I was the classic bright, awkward girl and I was massively out of my depth, only the very pretty, strong-willed girls really thrived. And yep, it did scar me for life.
By contrast I found my girls's school, despite being notoriously competitive, much more supportive. I had a niche there and I was very content. Despite leaving when I was 16 some of the girls I met there are still great, great friends. Eccentricity and being clever was encouraged, whether or not you were popular with boys was irrelevant (they weren't there to impress) so the girls were less inclined to suppress their natural selves and become identikit boy-pleasers. As motherinferior says, power of wimmin 
I'd happily send my daughter to an all-girls. I'd happily bomb the boys school I went to.