ok cyclone.
SS does take additional power to turn. I hate to be the one pointing this out... But your own post is a contradiction in itself... Don't see why it would take more power to turn a heavier material?
Think about it. A diesel engine --which typically has a much heavier rotating mass than a standard gas engine of similar size-- needs a much higher torque starter to get it turning... Additional battery amperage is also needed. Based solely on that logic, would you not agree that more power is necessary to turn the heavier rotating mass of the SS impeller? If this wasn't true, why would the companies making impellers, or for that matter, propellers constantly strive to make their product lighter but still maintain tensile strength?
However, you are right in one sense. Paying additional greenbacks to regress to an aluminum impeller would seem somewhat ridiculous.
Stick with the SS, use the money you would spend buying an aluminum impeller to make at least another 50 HP out of your Big Block...