Larry I tend to agree with you. Here is my take on it from a "Layman's" stand point. I have no pressure taking equipment however. "Max" pressure will be reached within the chamber proper itself. In fact the 1" location for the pressure recorder was chosen many years ago to insure it was close enough to the breech to always catch the max.
Assuming then this small gauge insert has internal dimensions proper to the gauge I can see no reason the "Max" pressure would be reduced, however at this point extra metal has been added to the chamber wall.
As the charge moves down the bbl pressure falls due to the expansion of the combustion area. In the case of the inserts as soon as it leaves the insert expansion will be greater than would be the case in a bore normal to the gauge, thus I can see no reason at all that pressure would not fall from that point forward.
