[aerohead]

The upper elongation moves the upper separation point back along with deceleration and pressure recovery.That's good.
The base pressure of the wake is however decided by the pressure at the taillights and below,which is well below that of the upper body.The low pressure below is adulterating the higher pressure up top,to compromise the performance of the existing elongatioon.
The only way to really cut drag is to enclose the entire perimeter,like a plan-tapered box-cavity which,combined with the upper mods, move the entire separation point rearwards with plan taper.
Something like a swing-away spare tire holder as used on Nissan SUVs,mounted to the bumper,would allow the lower portion to swing sideways for hatch operation.