[Artemis]

Interesting.

Personally not convinced that BMW M will use an 'add-on' trunk spoiler for the M2 CS and M4 CS. Keep the M Performance rear spoiler for the 'regular' M2 and M4.

No, try imagining the rear with an elegantly arched boot lid à la E46 M3 CSL. The camo on the M4 CS spy shots seems to hint more into that design direction. Any such (functional) cue may be as well, on the one hand, another way to further distinguish the more expensive CS model from a 'regular' M2 or M4, and, on the other hand, be more subtle (read: less extreme) than an in-your-face wing (let's keep that for the GTS/CSL models).