But do you redline your Z4 on a daily basis? When commuting I don't generally go above 3k rpm and use gentle acceleration. When I'm just going somewhere at the weekend, most of the time I'm hovering around 2k rpm for extended periods. Fact is turbo cars in general use tend to get better mpg than NA simply because to get the same kind of actual acceleration you don't need to climb up the revs so much. This is especially apparent on motorways where diesels and turbo petrols can change speed relatively effortlessly while NA you're more likely to drop a cog. A lower capacity also generally means more mpg when cruising.
Since these engines won't be replacing 6 cylinders until maybe 2013 at the earliest, I think they'll be coming in at just the right time. At the moment fuel is £1.33 a litre, based on the way it's been increasing I wouldn't be surprised if we're pushing £2 a litre at the end of 2013 - it'll take a doubling in economy just to keep the fuel bills the same as they are now.