I am not one bit surprised at his first observation, in fact, have "preached" it for some time. Essentially if you push the same amount of shot to the same velocity you have done the same work or approximately the same "Average Pressure" over the length of the barrel.
The So-Called Low-Pressure loads have not truly lowered the total pressure, just redistributed it to give a lower peak with a "Slightly" higher pressure spread out over the rest of the barrel. When smokeless powders became common the Gunmakers knew what they were doing, they Beefed up the chamber area of their barrels to contain these higher pressures.
I think you may have misread the part about the recoil. Higher pressures are the result of faster powders, not slower ones. assuming the same ballistics. I have never been much impressed by many of the so-called Felt Recoil statements. Most, in fact virtually all, simply do not have enough certified data to defend the statement. Data for the loading manuals are generally taken from new hulls & fired through a test barrel having minimum dimensions to ensure a reloader if he follows the "Recipe" will have no higher pressure than what is listed. He may well have lower, which also may result in a slightly lower velocity than listed in the manual. A load using a slow burning powder, taken out of the shot weight class for which it was truly developed, is MUCH MORE apt to be adversely affected.
For most of my older guns, I generally try to stick to loads of around 8K, unless they are true Light Breeched guns made in the strictly Black Powder era. In that case, I stick with Black or at least very light loads with which I can bring the pressure down to around 6K while still using a fast powder.
My personal opinion is that the slow powders were optimized for either heavier shot loads, higher velocities or Both & should not be used at pressures lower than around 8K. The slow burn characteristics are achieved either by a varied shape which deters burning, a heavier deterrent coating, or both. Neither of these conditions is an asset to efficient burn at low pressures.
