To get back to Doverham's original question.
All else being equal, lower pressures put less stress on a gun's jointing than higher pressures. Pressure acts in all directions & higher pressure pushes the case head back against the face of the action harder than lower pressure & this increases stress on the jointing. Given small increases in pressure the difference is likely minimal but it is still there so within reason, lower is to me a good idea for gun longevity.
Read Vic Venters comments in Gun Craft chapter II "Jointing & The Circle" on how modern gunmakers deal with the higher pressures encountered with double rifles.
I agree that assuming equal recoil & gun weight increased pressure provides no or very minimal increased stress on the wood & that you need high enough pressure in a load to provide reliable ignition & combustion in cold weather.
For years my main 12 ga target load has been a 7/8 oz load using PB at a published pressure of just over 6000 psi & it is extremely clean burning & I've never had any cold weather issues with this load
