If you really want to know how much sound a string can put into the air by itself you would have to tie it to a solid concrete support. The other common misconception is that an acoustic instrument somehow amplifies the sound of the string. An amplifier is a device that multiplies the power of a signal by drawing energy from an outside source. In an acoustic instrument, the string is the driver, there is no other source of energy for the sound. Once you pluck the string you have put in all of the energy you will have to work with until the next time you pluck it.
Most musical instruments are terribly inefficient.
Harps probably fall into the same range as guitars, maybe a little better, although I don't think the measurement has been made. We get away with it because we can feed in a fair amount of energy and because our listeners can get by with so little. When you are operating in these ranges things get hard to measure, and detail changes can loom large. One nice way to look at it is to stand the numbers on their head. That is one reason it is worth doing things like thinning the soundboard edges and sides, and choosing good wood for the soundboard.