Sorry for the late reply, but I'll put this one to bed!
Most cars produce high rear lift. A spoiler can and often does reduce rear lift, either directly on the spoilers blade, or via upstream effects:
Direct: a wing shaped cross section can generate low velocity at its tip followed by high flow acceleration on its lower edge, reducing static pressure and proving a downwards force. The same can apply inversely to the upper surface.
Indirect: the spoiler blocks the flows path and causes stagnation. This increases static pressure on the surfaces ahead of the spoiler- the boot lid and rear glass. Higher static pressure here pushes the rear downwards. This is the most likely gain from these aftermarket parts, the wing cross sections are usually shite from tuner parts!!
In terms of drag, it's hard to say the effect, the balance of upwards and downwash behind the car affect the way pressure is recovered behind it and can lead to large or smaller wakes
Source: I'm an actual aerodynamicist for Bentley motors