Been doing a lot of miles recently in my 2.4 due to a sick relative; and am actually very pleased with the fuel consumption - 38 to 42 achievable farly easily over a fast (but not frantic) cross-country route. I tend to overtake everything in sight when the opportunity arises (flat in third usually), but when following other taffic, short-shift at about 2,250 revs. Figures are based on my trip computer, which I've always found to be extremely accurate. In fact my range for this tank-ful has been showing as high as 675 miles. I think it made a big difference the fact that it had done a regen just before I filled up. These engines really really hate short journeys; but given a decent run; for the performance on offer, I don't think they're that bad on consumption.
Unfortunately, for me therein lies the blow that's going to probably see me and the Alfa part company shortly. Recent circumstances dictate that my mileage is going to much curtailed shortly, and with the 159 going out of warranty soon, and the expected DPF/EGR problems associated with short runs, I think much as I've loved the experience, it is best to let somebody else enjoy and use her properly.