If someone drives in a "normal" manner it wont matter what mode the car is in. I have spent a good deal of time in my job field with fuel managment as a UH-60 Blackhawk pilot. Since "normal" driving operation is only using a small amount of power which never approaches full open throttle/workload, mpg should not be affected and if it is it should be very minor. Really it all boils down to how aggresive you drive.