Chuck;
I am indeed following what you are saying. However, think very seriously about this question before replying.
Again looking an an old Alliant guide, I find two loads both with 1 1/8oz shot at a listed 1200fps. All components are "Identical" Except the wad. one used a Rem Fig 8 @ 10,700 psi, the other a Rem RXP12 @ 9,800psi. Now again think this over "Real CarefuL", can you "Positively" state the load using the Fig 8 accelerated faster, because it showed the highest pressure. "OR"?? did the Fig 8 wad take a tighter hold on the walls of that particular brand of case for whatever reason & Actually "Retard" the acceleration, creating the higher pressure. Remember "All" factors have to enter the equation. I certainly do not find it true that acceleration can be figured from the max pressure recorded in a pressure gun, nor for that matter, even from the entire pressure curve. Certainly a factor, But "Not" the only one.
I also think since the majority of plastic wads are rather loose in a bore & depend upon obturation for a seal, if not given a quick enough blow to ensure obturation, might quite well move down the bore with faster acceleration than if a faster powder was used giving that tight obturation, resulting in greater friction & keeping the pressure up but retarding movement. Many Many factors to be considered. I really think the only true answer lies with an accurate Accelerometer test of the gun's movement itself, while firing different loads.
Actually having followed your posts here for some time, I must say I am quite surprised you have seemingly failed to consider any factor other than just the pressure. This does not seem compatable to your usual thouroughness of a subject.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra