I think the point here which someone elluded to is the entire damn premise for M going turbo was efficiency!!! Without better efficiency than the benefit of the turbo is an absolute farse. So we get lag, low redline grunt monsters and no better mileage. I agree they are seeing no better mileage than hard driven e90 m3's and very mildly better than e60 m5's.
This is really big proof that M could have left the M department ALONE and stuck to their bread and butter, high revving race enginess and maybe lost 1-2 mpg on each car average in the M department which would not have any effect on fleet wide numbers.
And as owners say, what M buyer gives a crap about mileage? So at the end of the day the reason for going turbo and the benefits are really none. Low end torque wooo hoo