Your Insights roofline is right at the ragged edge of attachment
Not as bad as the 1st-gen CRX,which I tried to correct
You can see how Honda lofted the roofline of their 2nd-gen CRX,netting Cd 0.29.
With the flow attached all the way to the rear,adding the elongation brings the wake area down and base pressure up for an overall drag reduction,as you see the the Mrecedes-Benz C-111 III
Theoretically,your drag potential would just be a function of the amount of elongation.
An idealized aft-body would allow the bodies drag to approach the drag of half-bodies,and with wheels faired,perhaps Cd 0.12,maybe a tad lower
The drag trend would follow as in this table for a streamlined half-body with wheels in ground proximity
