Admittedly I do have a strange whoooshing noise from the passenger footwell when changing gear. I've heard that can be to do with a knackered DV. Don't know if that would contribute to poor mpg. My car isn't remapped and given the higher mileage (100k) I'm not sure I want to go down that route.
My average drive to work consists of 'getting through town a, driving down motorway, getting through town b to work'. Maybe it's the town driving that's killing the mpg? I certainly don't do 140mph down the motorway. Closer to 80-90 at best.