I agree that the real problem is not luxury sedans in terms of fuel consumption and excess use of resources. It's the fleets of commuters in SUVs and pick-ups that are also using 17L/100km, or more.
I think we need mandated fuel economy standards that are very high and guzzler taxes that are pretty much obscene. In the flagship luxury class and high end sport cars, adding $10K or more in fuel consumption taxes would be a small part of the price. If you were buying a $35,000 high consumption SUV and you faced a $10K guzzler penalty, well, that's another matter.
I tell you that F150's would suddenly be available with some better options in terms of economical drivelines and smaller pick ups would make a come-back.
on topic...
In this price class, I like the BMW, but in all honesty, they're all too big. The E, 5, etc. are the "right" size for a big luxury car to me.