[aerohead]

It's really about 'polar moment of inertia' ( flywheel effect ).
If a 'heavier' wheel has its mass concentrated closer to its 'center', than a 'lighter' wheel, it could be dead-heat!
I think Stellantis is offering an $11,000 set of wheels for the Dodge Challenger SRT Demon 170, which are carbon-fiber 'rims', bonded to aluminum 'spokes', held on by titanium lug nuts. Extremely-low polar moment.
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These same wheels would gut your mpg on the highway though, as they'd have as high an aerodynamic 'windage'-'ventilation' drag as you'd likely find.
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Even if you did nothing but stop-and-go driving, and considering the overall mass of the Sienna, I don't know that a lighter wheel would even move the dial.
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On a hybrid, plug-in hybrid, and BEV they might even reduce efficiency, as regen could suffer from the reduced inertia/momentum.
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If we had Toyota's numerical modelling software we could resolve the issue virtually.