Well then, I will be the first to try it. Adding a top tube to a frame I already know well should provide good evidence.

I should clarify that what I looking for is not more stiffness in the sense of the bottom bracket moving side to side when you mash on the pedals, but less noodliness or twistability in the front triangle. My front triangle gets really twisty when I make quick steering adjustments...which has been common to every large standard diameter steel tube frame I've ridden.

Incidentally, my reasoning for the double top tube is that for a large frame (say 63cm and up), you don't really have a front triangle.... you have a 4 sided shape. The difference between a triangle and a four sided shape is that you can twist the latter. The longer the headtube gets, the more it can twist. By adding the lower top tube, I am thinking that you essentially create the less twisty front triangle that short frames have, but still have the height you need.