[aerohead]

There may be data out there which might allow you to predict what the difference would be.I've no idea how one might access that.
It would be a very expensive proposition to build a test model for actual testing.
And I've no idea how you'd construct the spherical coordinate system 'cloud',CFD would require in which to test it virtually.
One consideration about the NACA/Morelli shape,is that it is inferred by the report on the 1978 CNR/Morelli/PININFARINA 'banana' car,4-passenger study;that adding the tail back to the car would NOT reduce the cars drag in any meaningful way,which,if true,would 'freeze' the drag at Cd 0.23,with no hope of going lower.
Perhaps a reason that no automaker stepped up to the plate,ready to pay a licensing fee for which to commercially produce the shape.
Also,within a few years,Pininfarina produced lower drag cars using more 'classic' streamlining tools.