I agree with all the above. There are SAAMI specs--voluntary guidelines for the industry--available for only a few "+P" considerations--most notably, 38 Special and .45ACP. Those specs originally were developed for 'early' quests in better hollowpoint bullet use, or in use with heavier bullets used for hunting.
Unfortunately, the SAAMI specs have also been tweaked as the years passed--downward. Where +P recipes come in to play is in the 38+P PD loads--i.e., getting enough velocity to provide adequate penetration for a medium-weight bullet that will expand nicely (specifically, the Speer Gold Dot Short Barrel bullets). And, even there, in building 'replica rounds' based on Speer-developed recipes, one needs to use loads that are MAX loads based on Speer's 20,000 (psi) rating, NOT on the current downgraded SAAMI spec of 18,500.
I'd shoot these 38+P MAX loads in my +P-rated .442 and feel secure in their performance. With AA#5, they get into the bottom-level velocities Speer wants for that bullet--i.e., above 850 fps.
But, I'd much rather shoot the same ballistic performance in a "357 lite" load--that's one translated from the Speer 38+P recipes--in a 640 38/357 j frame--if only for the obvious safety margin (20-22,000 in 38 Special is pushing hard; in 357 Magnum, it's a low pressure load) and for the cylinder-crud issues.
Jim H.