Especially with a new belt, much of that crud is in fact from the belt. New belts have a special coating that protects the belt and sprockets during initial break-in, and it wears off rapidly and gets deposited on the wheel and the mufflers. It's part of the reason why a new belt needs to be readjusted very shortly after being put into service, often 500 miles or less.
As for the actual brake dust, I switched to Lyndall Z+ pads on the old SEEG and saw a huge improvement in visible brake dust on the wheels. The stock pads on those older bikes seemed to be made mostly of carbon black filler, and they would turn a wheel black in no time. I'm not sure if the difference would be so pronounced with an '08 or later bike with the current brake design (Brembo).
Jerry