The lenses are advertised as 97% efficient. Whether this is entirely true or not I can't say but when factoring in the fact that most of the light doesn't touch the reflector and they are 93% efficient then the total output should be around 99%.
I've been running cobs optic free for 2 years now, not a spot of dust. They seem to repel particles. This may have something to do with the photon output but I suspect there is a bit of laminar flow produced by the temperature differential. My experience has been with passive heatsinks and vertical air movement so the flow pattern will change, but I still think the LES will stay fairly dust free over the long term, especially with reflectors in place.