Here we go
i found severel test from the german magasin sportauto
first rs6 salon vs e63 amg, then m5 vs cls63 amg, rs6 avant, and supertest of m5 (3years ago)
you will find out that rs6 salon and m5 are pretty even concerning acceleration and braking (both test from this year)
and aswell e63 and cls63 amg are pretty even.
RS6 avant is a bit slower than rs6 saloon and m5.
M5 supertest was a massif time, but maybe cheated by bmw sending a prepared model.
the rs6 salon supertest will be (or is already hold with the example, (red saloon) above.
enjoy (klick on picture to make it bigger)