I think it goes back to the blank. A great 2pc blank will be more sensitive than a crappy 1pc.
That said, I really don't think the number of pieces is an issue anymore. The standard fly rod is 4-piece, and you can even buy 6 and 7-piece travel rods. Most of the major fly companies don't even make 2pc rods anymore. Ferrules today are light years beyond what they used to be.