I agree that the effect of tires is not really that significant. My initial reaction would be that, in general, the RFTs might provide better MPG due to the stiffer sidewalls, in spite of heavier weight, as a lot of the rolling resistance has to do with sidewall deformation as the tire rotates, hence the reason higher tire pressures give better MPG.

As you note, the tire compound also impacts this, with the softer compounds typically having more rolling resistance.

In those MPG contests to max-out MPG, they generally go with hard compounds and high tire pressures.

But if one is buying a BMW, I would expect getting tires with good grip and handling would outweigh a few percent in MPG, making it a moot point for most BMW drivers. A change in driving style would provide much more MPG benefit to most drivers, as compared to which tire they choose, e.g., anticipating stop light changes, smoother and lighter foot on the throttle, etc.