Question part two: Why is it that many sprint/TT/pursuit/tri etc bikes tend to have a lowered/shortened seat stay?
when you talk about compact and the top tube, do you mean that the top tube is sloping and not parallel to the ground?

I was checking out a used frame that was as you described: "Sometimes it is claimed that rigidity is enhanced when the stay joins the seat-tube low and then joins the top-tube several inches ahead of the seat=tube". Out of pure boredom with the typical road frame geometry, I was thinking about building up a frame like this.