Sorry, I can't help much. I went to Mayfield, though years ago, and it has changed a lot since my day.
I have been back fairly recently and hear a bit about the school from a friend who knows one of the Governors, and via OG networks.
Like all schools it is about fit. I would certainly recommend the school for the right child.
Catholic schools are often interesting in that they tend to take a broader range of abilities, so don't fly high in the league tables. It also means that they attract a wider range of overseas boarders: Spain, Latin America, Poland, Nigeria, along with Asians which helps prevent cliques forming, an issue elsewhere. Mayfield has always been socially outranked by St Mary's Ascot, which I think is a good thing. Mayfield girls don't seem to be back in London every weekend to attend smart parties. The religion sits lighter as well, presumably diluted by a number of non-Catholic day girls, but the basic ethos remains.
Given its broader intake Mayfield's results seem impressive, especially in maths/STEM. They will get the academic child to Oxbridge if that is where they should go, but without neglecting the others with different talents and abilities. My own DC went through selective London day schools and though they thrived, I see real value in schools where you don't have to be clever. At Mayfield recognition can be earned through its strong singing, pottery and other creative arts, horse riding etc.
This impression was reinforced by someone telling me on my last visit that they are seeing increasing numbers of applications from ordinary London parents, not the super-smart set, but those who want a gentler, more nurturing, education for girls that is not easily found in London day schools. Mayfield is perhaps a school that sails under the radar for girls who themselves might sail under the radar in a more robust environment, but one with real excellence in academics, sport and creative arts.