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Also remember rotational inertia. Most of the weight of the rim is pushed farther out from the center of the wheel. While you may have saved a # or two, it's still just as hard if not harder to turn the wheel, regardless of how close to the original diameter.
Here's an experiment to show what I mean. Hold some weights close to your body and spin around. Now reduce the weight by a little bit, but hold your arms out with some weights in each hand. You got "lighter", but was it any less difficult to spin? This is why I'm looking in to the light weight (but cliche) rota slip streams in 16".