I can add a little more information - but, I'm afraid, no real knowledge of the school/stream.
Five years ago RGS federated with Kendrick & the Kendrick became exec head of both schools, this was to raise standards at RGS (but Kendrick also got a chunk of funding to build a new 6th form block).
I don't know what effect this has had a RGS. RGS girls regulary visit Kendrick, I have a DD at Kendrick, who says the girls are all friendly/keen to learn etc but very suprised at Kendrick lessons, for example, when they are asked to discuss something they do & don't chat, lessons are quite quiet focused etc, all this comes as a great suprise to RGS girls, which doesn't suggest good things about the school to me.
The upside is that, I think, one of the conditions of the new 6th form funding was that this should extend to RGS girls. Kendrick take additional girls into 6th form (I think approx extra 40 girls) and a certain number have to come from RGS, I don't know any of these details though.
This probably hasn't been a lot of help - just had a thought, though - you could try visiting Kendrick Open evening & question an ex RGS 6th former!