FWIW, I've got 9000 miles on the 520 DID chain and AFAM sprockets--with the hard anodized aluminum rear--on my 600RR, and both chain and sprockets are still, literally, like new. I just adjusted the chain--a hair--for only the third time in those 9000 miles (h*ll, the chain quides still register in the "new" range), and the teeth on the rear sprocket have just barely worn the anodized finish off but show absolutely no other wear.
This is my 3rd 600 running DID/AFAM 520 set-ups, and they've routinely matched/exceeded the heavier stock set-ups in terms of longevity. And they've seen race duty in addition to weekly up-tempo Sunday rides and extended high speed touring.
However, a number of aluminum sprocket manufacturers don't offer a hardened product like AFAM and others do, and cleaning, lubrication, and maintainance are more critical with an aluminum sprocket than with steel. But, with a quality product and close attention to proper care, they'll last just as long (on a 600) as steel. :waytogo: