Salisbury use their choristers fairly equally and make a point of not advertising which choir, boys or girls, is singing a particular service or broadcast. Big services are joint. When my daughter was there I couldn't always tell who was singing at advent services where they often alternate for different carols as they move around the cathedral, even though I knew a lot of the voices well.
Some of the cathedrals have deliberately used a different age range for their girls which helps keep what the boys are doing distinct (so they don't lose the boys); the presence of older girls tends to change the overall sound, but also an older group can approach repertoire differently.
I've always felt that at least some girls are capable of singing with a "true treble" sound - first became aware of this hearing my younger sister sing a spine-tingling solo at a school carol service age 12. Equally boys often aren't trained to the pure clear "traditional" treble sound (and I'm not sure what I think of that). Oh and girls do experience a voice change around puberty too, just much less pronounced so unless they are singing a lot you won't notice it (my daughter lost about a third off her "top edge" around age 11 for example).
I get the impression there is a creeping acceptance of women altos into traditional choirs.
headinhands you mention Wells - they are a cathedral that runs boys and girls lines; we considered looking at it before my daughter went to Salisbury.
Personality and socialisation differences influence how choristers respond to training - it is interesting to talk to choirmasters who have charge of different groups about how differently they find they work with boys and girls, even though with the same goal. The "dynamics" of a mixed group could be different again. I'd hate to see all boys choirs disappear in favour of only mixed ones, there is a role for both. Finally, the elephant in the room with the college choirs that still have boys, is the choirschools. Those that remain boys only are often not big enough to expand sufficiently to take girls. If you train choristers without a choirschool, logistics make it impractical for them to sing so many weekly services, and their overall rehearsal time will be much less. (I think my children had 10-12 hours a week actual rehearsals, not including services times).
I'm not yet familiar with the fine details of university choral scholar opportunities, just starting to research it as my daughter will certainly be considering it over the next couple of years.
In terms of overall musical achievement, the thing that makes the biggest difference is the expectation that fairly young children can make music at a high level. If you expect that of singers then you get amazing results. If you expect it on instruments, you can also get amazing results. If you start later and slower and think only a little time should be given over to music practice, then you get slower results. I'd like to encourage everyone to find ways to pursue music with their children, surround them with a range of good examples, get them involved in music making, believe in them and encourage them to put in the effort and go out and join groups where they can grow their achievements.