I just exported a playlist group. It seems to glue together all the individual playlists into one big playlist file.
I then tried exporting all playlists. As you postulated, that exports everything as individual playlist files. But the playlist groups become folders in the file system! That's very convenient and allows you to very easily delete everything you don't want and keep the group you do. It's not super elegant, but seems easy enough to achieve your goal.
Good luck,
Brian.
