Well, a friend of mine recently installed Akra full exhaust for F10 M5 on his F06 M6. The fitment wasn't as easy as expected.
It turned out that the geometry of the F10 M5 exhaust is different from the F06 M6 exhaust. Some slight modifications had to be done from the inner part (close to the downpipes) for the exhaust to be fitted properly. It does extend outwards slightly after the installation, however, with the right diffuser it'll look good.
Pics below:
