Hucho states emphatically,that the lowest drag is always achieved with the fastback roofline.And it needs to follow closely to the Mair/Buchheim contour.
Here,GM has used it on their Citation concept,which has returned Cds below 0.14.
A recent Buick Riviera concept
Here is Mair
Here is Buchheim
The Aerodynamic Streamlining tool at the top of the forum page respects Mair's contour.If you wanted to consider a Kamm extension,that's what it's for.