Brake dust is that filth that uglifies the wheels of those of us who use the cheapy brake pads.
I just refinished the Coupe's alloys and took pains to sand the backsides of the rims down, paint them (and the bead areas), and wax the living **** outta them front and back. Hopefully the filth will wash off more easily.
Oh my such beautiful wheels- the ones near drums will stay this way much longer. <3 Wish the rest of the car matched.
Anyway re: guards helping aero: I think they should in theory; in practice I doubt you'll be able to detect the difference.