i am getting 25 mpg in my 540 on hwy miles, about 20-21 when in the city. Not too shabby in comparisen given the power diference.
Are you running premium fuel?
have you had the plugs changed since you purchased?
Is there any pinging or excessive emissions or other unfriendly sounds while pedal is hard down?
What is your tyre pressure.
Just a few things to start with.