Note the above posted pressure curves. Note these would not in reality start at 0" as the shell would have a base wad, but note how high the pressure rises before hardly any movement of the charge itself takes place. Also note this initial movement of the charge is a very critical factor. Too soon & pressure will not rise quick enough to insure adequate combustion & a "Squib" may be the result. Too late & pressure can Spike to a dangerous level. Way too late as in putting a stiff charge of a fast powder behind a heavy shot wt & the charge might not leave the bbl at all. In this case the shooter would experience hardly any recoil at all, but most likely would note the "Blinding Flash" as the chamber top lifted off in front of his face. (Very High Pressure)
The question boils down to exactly how "All" factors interact to affect the acceleration of the charge down the bbl, & exactly how that acceleration affects what the shooter feels. I am very willing to recognise that "IF" that rate of acceleration of the gun can be showed to vary to a significant degree, that even though total recoil developed remains the same, there can be a difference in how it is absorbed by the shooter. In this case I Would be in the camp who believed the "Slower" rate of acceleration would be the most comfortable to fire.
I am at still however, this point thoroughly convinced that within the parameters of currently available shotshell powders suitable for consistent reliability for a given load that not enough variation will exist for a shooter to seperate two loads into "Mild" vs "Brutal" by the firing of a few shots. Possibly there might be a slight difference in "Accumalated" fatigue by the firing of a large number of shots over a relatively short time. In this case extreme care will have to be excerised to insure one is actually testing the different loads & not "Themselves" over different sessions as alluded to by another poster, several pages back.
This is esentially my stand on this & I am "Always" open to any "Meaningful Input".

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