add the rear under body diffuser by buying the parts? (not the spoiler, but the under body panel)

I can understand if Infiniti packaged the two together, but I saw the aero package (slightly different, now with front chain spoiler) is also available (from the Infiniti website) as an dealer installed "accessories package", but yet, when I select that on the website, it would again state that I need to add sun roof. (Which is kind of strange, consider that it is a dealer installed option..)

So the question is, can I get a base 6MT, no sunroof (I never use it..), and still get the rear under body diffuser for the rear 0 lift at speed?

Do the rear diffusers actually work? I looked under the car and they don't look like they do much. Or do they actually grab that much air?