DaveL, I will agree with you that once the o-rings wear out on an o-ring chain, you might as well throw in the towel. The o-rings are responsible for "lubing" the chain. If you keep the o-rings properly maintained, this chain will last just as long as any other (yes, eventually everything will wear out). The problem with that chain occurs when o-rings stop performing as they should, just as you mentioned. They usually become dry and cracked from improper lubrication and or neglect. I've also seen people using petroluem/mineral spirit based products to "clean" their chain. This will lead to the o-ring breaking down and ultimately being worthless. 15k for an o-ring chain is unfortunately the norm . . . but properly maintained it will last a lot longer. (my instructor has 30K on his SV and it's still good)
As for the aluminum sprocket, we can agree to disagree. Granted, common sense would tell you that you don't want to use a lightweight metal when dealing with a D9 weighing 50 tons. (it would kinda defeat it's purpose) The cam sprocket is not really going to give you any mechanical advantage if you lighten it. It's main purpose is to last . . . a long time. Besides, the weight saving potential isn't really significant, or non-existent depending on your application. Neither really is the rear sprocket, but then again, what's the old cliche . . . every 1lb = 7 horsepower. If I'm going to change out my chain, I'm usually going to simultaneously swap out my sprockets. IMO, that just makes sense. It's been my experience that by maintaining both I can get the max life out of them.
Aluminum is lighter and softer. It will wear more quickly. Yes, I don't need to be an engineer for 6 months to tell you that. I was assuming by the original question posted, that he was converting to a 520 chain to save final drive mass. To convert, he would need to buy all new sprockets. If I'm going to replace everything anyways, and my goal is to save weight (why else would I be converting to a smaller chain), why not go with the aluminum rear?? For me, the sprockets and chain go hand in hand. I've never had one wear out significantly more than the other.
Unfortunately you cannot have the best of both worlds. If so, then we wouldn't be having this discussion. Do you want reliability?? Or, do you want performance and that hole-shot?? Consider the pluses and minuses and decide what is best going to suit your particular application.