I don't know if they are underplayed, but each one of them is for me a marvel to listen to, and I intend to play them some day. For a very good recording, look for Maria João Pires who's got not only that set of impromptus but also the Op. 142, if I'm not mistaken, and some other small pieces.
I think the CD is called 'Le Voyage Magnifique'.
Alex