There's a reason the M5 diffuser only fits the M5...
Without modification you're not going to fit it. Now that I have my Akrapovic exhaust I might sell my OEM M5 exhaust.
2014 X5 xDrive50i ZMP - Alpine White/Mocha (On Order)
2013 Tesla Model S P85+ - Gray/Black (Sold)
2013 F10 M5 (Sold)
2013 Cirrus SR22T