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You're probably correct that it's not as big of an effect as the piston skirt/ring friction, but it costs a non-trivial amount of power.
You want to reduce throttle position if you're under light load, because that equates to less fuel burn, and a higher gear will almost always let you do that, the rare exception being when going up a steep hill or maybe with certain turbo engines. Remember, if the engine speed is lower, the volumetric efficiency at the same throttle position will be higher, because the pressure drop is lower across the same throttle plate if the air is moving by slower. However when you are accelerating, that is the perfect chance to open up the engine and operate it at higher efficiency levels, so you don't want to feather the gas while accelerating.
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