You're defining "high pressure" differently.
I am not defining anything.
SAAMI defines what the MAP is for each cartridge not me.
SAAMI defines that a 40, 9mm and .357 all have a MAP of 35,000psi.
SAAMI also defines that a 9mm+p load operates at 38,500 psi.
SAAMI does not define a pressure for a 40+p nor a 9mm+p+.
Apparently after doing some reading it appears the few "+P" loadings are ABOVE max.
All +p loadings are above MAP set for a standard loading. A proof load is 130% of the standard load. This is what is determined to be the limit of the cartridge. SAAMI only lists +pressure loads for a few cartridges but for the few that it does... a +p load is 10% above what is set for a standard load. As there is no official +p+ load all one can do is guess that is somewhere between a +p load and a proof load. Probably 15-20% above a standard load.
Conversely, the other loadings you mention do not typically operate at max.
How do you know what each cartridge operates at?
Do you have access to test barrels and a piezo equipment?
So in essence it would be o.k. to shoot .38 special +p ammo out of a .357 magnum as the pressure would be close to the same as a magnum round or at least within the safe limits,right?
This is correct. All 38 special loads are at lower chamber pressures than 357 magnum loads.
Here is a short list of the more popular cartridge pressures as set by SAAMI.
http://www.leverguns.com/articles/saami_pressures.htm