I prefer to clean my cars at home/dorm, not during race nights -- don't wanna risk getting dirt into the very vulnerable ball differential. Having a bad diff alone costs me 0.15 seconds per lap! lol...
And yes, all that dust is straight from the RCP. The dust has a way of getting stuck in the "dead airspace" between the body and chassis, and it doesn't help that I have exposed grease in my damper tubes -- it just sticks and doesn't come off without a good brushing. After I clean it off my car will actually be a full gram lighter.
Dr. K, my GT-R is lowered about 2mm further than the stock clips will allow, and my chassis bottoms out on the ground just before the rear tires (24.5mm diameter) touch the fender. The fronts (23mm) aren't even close, I have so much room I'm going to run the fronts at 24mm next time I race (slightly more front grip).