[aerohead]

A full boat tail would drop the Subie's drag from Cd 0.405,to 0.12.At 55 mph you'd be looking at a 35% mpg increase.
Baron Reinhard von Fachsenfeld/Professor Wunibald I.Kamm both knew the value of a full boat tail and these are included in all their published research.The K-form was just a compromise and concession to length issues during urban driving.
A 50% truncation will not give the drag reduction of a full tail but it is certainly easier to 'park,' and get into a garage.
To address the length issue of a full tail,Fachsenfeld patented the inflatable boat tail which you can see in Hucho's book back in the commercial vehicle section.
NASA's boat tail helped reduce the drag of their Ford Econoline from Cd 0.48,to Cd 0.235.Which leaves half the drag still on the table.