My hunch is the increase in track and wheelbase for the new three series is to bump it up into the next larger vehicle footprint class for US EPA mileage rules. the 8 speed automatic makes a lot of sense as it gets great EPA numbers in the 535 GT. LIkewise, replacing the normally aspirated 6 with a turbocharged four should do wonders for gas mileage with no horsepower loss. The four should also weigh a bunch kess than the 6.
Currently, the 3 is in class 2 which is for subcompact cars. Class one is for mini cars. If BMW can get the dimensions into class 3, the fuel economy target is nearly 10% lower than class 2. That means N55 engine availability and the associated performance. I did the math and a 3" wheelbase plus 2" track increase would do the job. [By comparison the 1 series just makes it into class 2.]
Vehicle footprint is wheelbase x track, so it is advantageous for manufacturers to reduce overhang and maximize wheelbase.
In 1968 when pollution controls started in the US, the auto industry world wide was not ready with good technological solutions. Consequently, horsepower, fuel mileage and drivability took a huge hit and did not recover until the early 1990's. I just hope it's not 1968 again.