You'd be surprised at the performance gains to be had from reducing rotating mass. On a drag car the rule of thumb is 10 pound reduction in rotating mass = 100 pounds overall weight in terms of performance, how true that is I can't say.
From experience I can say that on dirt bikes a light weight chain and sprocket set is pretty much equal performance wise to adding an aftermarket exhaust and rejetting the carb.
Also modern aluminum sprockets are amazingly tough, they wont live as long as steel but they do last well if maintained.
