[DannoXYZ]

Design is a big factor here as well. You don't want to match 1:1 the shape of steel sprocket in aluminium as 1/3rd weight may not be enough. You'll want to have more material, even mostly solid aluminium sprocket for only 1/2 weight savings. That should be sufficient strength. In end, it's the teeth that will wear out faster and require more frequent inspection and replacement.

On my street bike, I use a lightweight steel 520 sprocket that's actually close to aluminium in weight-savings. I get about 1.5-years out of my 428 alloy rear-sprocket on my race bike with flipping it. I do like the 428 conversion though as the chain lasts longer than factory 520. And I can fine-tune final-drive ratios in smaller increments using 18/57t, 18/58t or 18/59t depending upon the track.