First & foremost note one thing well, "Maximum" chamber pressure is "Not" the final determination of muzzle velocity in any firearm. Pressure in any firearm is always in a Curve. Upon ignition the pressure rises very quickly & then as the charge begins to move giving it more area it begins to fall off.
Loads requiring more velocity or a heavier load require a slower burning powder to achieve the goal without reaching excess Max pressure.
To use your rifle experience as example if you note the charge tables well you will see that as the powders get slower, the charges are increased & can give higher velocities at similar pressures to a lighter charge at a slower velocity of a faster powder until the case capacity is reached where no more powder can be added. Thus a 300 Win Magnum will burn a lot more of slower powder & give a higher velocity than can be reached with a .308 Win even if they operate at the same max pressure.
To put it briefly & simplistically, it's the overall "Average" pressure of the entire curve & NOT the max pressure which determines the velocity. I have never seen it demonstrated but understand that it would be possible by use of an extremely fast powder such as a detonating powder to actually burst the chamber of an ordinary rifle without actually moving the bullet more than a minute amount, the pressure simply rising faster than the bullet could move enough to relax it.
I realize this does not exactly answer your question, but hope it is of help in understanding how the pressure reacts. I will also note the recoil is not determined by the Max pressure either but by the overall ballistics of the load. Thus a heavier load or a higher velocity will give a higher recoil.
Although I have not done any actual chronograph tests I have serious doubts the firing the same load from both a 2 3/4" & a shorter chamber in the 2 3/4" hull is going to make enough difference in the overall ballistics that you could ever "Feel" a difference. There might well be a very slight difference in the pressure curve in the come area but not likely much in the overall average.
