EPA ratings are 18 city, 28 highway for her 2011 328i. 23mpg is right smack in the middle which is what I would expect with her daily route of 50:50 mixed driving. N51/2 328s really get pretty bad fuel economy considering the performance level.
She drives a little on the aggressive side but nothing extreme. I can coax an additional 1mpg out of the car when I drive it.
As to you getting 27mpg out of an N54 in "mixed" driving, I really doubt it. Your car was rated at 28mpg highway after all (assuming it is an automatic). You either have a factory freak car, hypermile everywhere you go or you are relying on the on board computer to tell you your mpg, which is always wrong in my experience. I determine fuel economy by actually dividing the miles traveled since the last fueling by the gallons it took to fill. Consistently less actual mpg than the car reports.