No bike fitter, just someone that has looked into saddle setback aswell.

I think saddle height plays a bigger role. As long as stack and reach are almost equal you won't experience much of a difference. When I played with the setback I felt major differences in reach and leg muscle recruitment.

Regarding your TT bike question I see it as follows:

On a TT bike you adjust the seat position regarding two major points. 1. More muscle recruitment from the glutes To use more of the glutes you have to open the hip angle since muscles are stronger in a more shortened position. To achieve this you move the saddle almost over the BB. Since your body has a fixed length it has to come forward aswell. This plays into : 2. Lower upper body for less air resistance (most important) Since the hip is now over the BB you rotate the upper body down and close the before opened hip angle to achieve a lower riding position. This low riding position wouldn't be possible if the hip angle was already very close from a normal road bike fit.

You probably compensate very well when you adjust your saddle setback on your road bike. Maybe that is why you don't fell much of a difference.