Then it would be an overdrive pulley.
The benefit would come from reduce rotational mass, but it wouldn't really be anything you'd see when you fill up. As far as alternator output, it would have more output at lower engine RPMs since it woudl be spinning faster. However, since it is spinning faster, the brushes and bearings would wear faster... Depending on how large you go, it may not be designed to handle the RPMs you spin it to if you rev up the engine.