My BTI airbox looked twice as bad as that when I got back to FL after 4 days of straight riding at windrock and royal blue, right around 160miles and not a spec of dust in the throttle body, So far the BTI air box seems to be really working.
I also used the that Black RVT gasket sealer stuff around the throttle body boot and throttle body, I also think that's a place dust is slipping through.