Certainly I would not expect much (or any) improvement during steady state driving, and yes, polar moment of inertia is what you want for energy savings when accelerating and decelerating. For this reason, the best inertial savings are too be found in the tires.
Most lightweight cast wheels save weight in the spokes and hub. It's only when you get up into forged wheels that the weight savings start happening in the drum.
On the other hand, improvements in ride quality happen no matter where you remove weight from the wheel assembly. A lighter unsprung mass transmits less jarring to the chassis, in large part because it weighs less relative to the rest of the car.
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