I wonder about this. Why would the mighty 5 series have to settle for the same tt the 3 series has? Why wouldn't it always have a more powerful engine as it carries a higher "premium" value? Or conversely why didn't the 3 series get limited to the 328 model and leave the more powerful tt for the 5?

It seems to me that the economics would dictate a fewer number of drivetrains. If the tt six is in the X5 and an uprated tt 6 is in the X6, unless it is the same basic engine with slightly different tuning for a bump in HP (Porsche does this), then BMW has to produce 2 entirely different motors, which based on history has only happened for a very short time until a changeover can occur.

Just my $.02 worth.