Don't sweat it, it is normal. The rears are used a small percentage relative to the fronts. It is only surface corrosion which will disappear during usage.I drove around pretty aggressively for an hour. It's fine on the front, but not all gone on the rears. it is significantly better, but not all gone. I guess I just have to do some more spirited driving
It will not increase braking performance unless the brakes are properly "bedded" afterwards.You can also take your Brakes apart and scuff the rust off the rotors and pads with fine sandpaper. That also helps to break the glaze on the rotors which Happens over time. It will increase braking performance.
I've had the opposite effect, wet wheel, spray, use soft brush and rinse, don't let it dry in the wheels and it actually cleans some of the rust out, maybe I've been lucky...Never use this crap. When I used it, it stained the fcuk out of my calipers. Good thing I'm getting them painted.