[aerohead]

I'm guessing a drag increase,however,if the backlight angle is around 28-degrees,the Swift would have bi-stable flow,alternating between squareback flow and fastback flow,creating a pulsing wake which the spoiler might help attenuate.
You'll notice sharp edges on rear bumper fascias of modern cars.In an aero-accoustic wind tunnel the original bumper may have been creating this pulsing,which is fixed by the 'burst' structure,fixing the drag coefficient,and lowering wind noise.