Energy conservation doesn't come into play, because unlike on a plucked instrument, the bow is there continually supplying vibration. If more of it gets emitted

Energy conservation doesn't come into play, because unlike on a plucked instrument, the bow is there continually supplying vibration. If more of it gets emitted, then more of the bow's energy goes into vibration instead of friction. On the other hand, with a guitar, the more efficient a radiator it is, the louder it is but the less it sustains, because each pluck imparts a fixed amount of energy to the instrument.