Mr. Borland, you've missed something:
But pressures are normalized to surface area, not volume (pounds per square (not cubic) inch), and the area of the surface being acted on (the bullet) isn't changed.
Pressure is being applied to the entire inside volume of the shell,
plus the back of the bullet. If pressure was only being applied to the bullet, well that would solve that whole "recoil" issue, right?
Look...the best data I've seen on what minor improvements to case volume do to pressures comes from John Linebaugh's data comparing the 44Mag with the 45LC in strong guns (where he started before graduating to the 475/etc.).
The drops in pressure are enormous. If I'm reading him right, a 32,000psi 45LC load will equal the performance of a 40,000psi 44Mag round - same bullet weight, same velocity, same wear and tear on the gun (minimal either way as long as the gun is in good shape and has tight chambers).
SAAMI spec on the 45LC is a bit over 15,000psi, so an 8,000psi drop in this class of ammo is more than "significant".
As to shell setback:
If the bottleneck cartridge is designed right and/or the transition isn't all that much, it's not a problem. The 38-40 and 44-40 had very gentle transitions from one bore size to the other. The 356GNR is supposedly not experiencing setback at all, probably because the transition isn't all that radical.
Early 38/44B&Ds were having a problem unless care was taken to have oil-free chambers, clean brass, etc. Bain and Davis altered the design to reduce the "abruptness" of the shoulder and make it look more like the old 38-40, and that solved the problem. Hornady's dies are of the "Mark2" variety. You need to make sure all your components plus custom cylinder are of the second type. I believe B&D calls this the "357/44" to distinguish it but Hornady doesn't yet.