As matt boy suggests, the D400 uses a center balanced valve not a pilot or servo valve.
The D400 also differs from side exahust valves in one other respect. The diaphragm angle combined with the coaxial exhaust valve results in the best case CGF scenario occurring in the normal smimming position and the worst case CGF scenario occuring in a position a diver will seldom be in.
So in short, the CGF effect is limited to no more than about .5", the occurs in the worst case position and in the normal swimming position, the effect is basically .0".
How is that different than a coaxial diaphragm or radial side exhaust? The D400 exhaust is about 2" lower, but the exhaust/diaphragm and center mass of lung are virtually the same in a near horizontal "swimming position" a few degrees up it is better, and few degrees lower it is worse.
Regardless, I don't understand your reservations about the statements made.
From your case geometry link, I believe we both agree that the pressure differential the body's air filled cavities are exposed to is determined by regulator depth, regardless of design. Exhalation pressure is determined by the depth of the high-point of the exhaust valve and flow resistance while inhalation resistance is a function of supply valve flow resistance and hysteresis. Any regulator position in a given position in the water has a neutral pressure.
I contend that the observations made by Onewolf in the original post cannot be explained by case geometry.
While conducting a test of running out of air in the swimming pool last week, I noticed that both my primary reg (Atomic B1) and secondary air (Atomic SS1) were much harder to breath when I was laying on my back versus any other position. I was curious if this is normal and what causes this regulator behavior?
Thanks.
Doug
I believe that Onewolf is describing his perceived increased work of breathing, which is a physiological measurement, caused by hydrostatic pressure differential between his respiratory center mass and airway. The experience is nearly the same even if his head were out of the water to lip level. That increased breathing effort is far in excess of anything that can be explained by positional characteristics his two different Atomic second stages. I believe that is valid since he didn't indicate any perceived difference between the bottom exhaust B1 and the side exhaust SS1.
I also contend that no regulator design currently or previously on the market would reduce that perceived work of breathing. To illustrate my assertion, I was part of a project to develop a lightweight commercial helmet. There are two demand regulator approaches. The most common is with the demand regulator's diaphragm exposed to water like the Kirby-Morgan gear. The second is to install the regulator entirely inside the hat and impart a delta P on the diaphragm between hat pressure and inside the oral-nasal mask or a mouth breathing tube.
Either approach effectively puts the diver's entire head inside the regulator housing, sealed around the neck. The second design eliminates all factors relevant to the case geometry fault analysis. Work of breathing is equivalent to standing upright in a swimming pool, with the water level around jaw level, and breathing from a regulator. The end result was the difference in the two designs was measurable, but too small to matter in situ.
The worst part of the whole exercise was the final report included a statement to the effect that lightweight helmets can never produce as low a respiratory work load as traditional deep sea rig/heavy gear without positive assisted breathing. I have an aging mentor from the deep sea gear era that still gives me crap about that. I suppose I deserve it for never giving him any slack about his Civil War "technology".
If we agree this far, please let me know where my statements were incorrect or misleading. If you do not agree, please explain the error. Nobody wants to put out bad information.
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Never thought I'd see the day. 
