I've never made or tested side skirts before. But I was emboldened by GM's 'Vette patent:
Thread:
GM aero patents (includes active side skirts for drag reduction)
The patent describes drag reduction in terms of shielding the rear tires (which, in the case of the Vette are probably 3 or 4 times wider than the Mirage's!).
The 14-15 Mirage didn't even get spats ahead of the rear tires. So hopefully this helps instead of hurting, because I won't be doing any A-B-A testing before April or May at the earliest!
I also like that the rocker skirt gives me options for integrating the rear wheel skirt -- could extend the wheel skirt lower and fair them together.
Also, while the middle of the Mirage underbody is relatively smooth (it has 2 OEM plastic aero panels), the rear underbody looks like a bit of a disaster. The side skirts should mean reduced flow there.
Next up: re-do the Kammback & air dam in coroplast.