Pressure is not linear, but the pressure curve profile depends on the properties of the powder itself. It is not the peak pressure that rips guns to pieces but how fast the pressure rises. Steel is elastic. it expands and contracts. If you build pressure slowly, the steel will be able to contain it more of it than if you build pressure more rapidly. Pressure vs time is what matters more, not the SAAMI max.
Lil'Gun in the 357 Mag has a very broad pressure curve. This powder builds pressure slowly and gradually. If you look at a plot of Lil'gun vs H110, for example, you'll see Lil'Gun builds 50% less pressure per grain. Lil'Gun max loads are not due to pressure, but due to the fact there is no more room in the case. It maxes out at 30,000 PSI. You can esily run 43,000 PSI in a conventional 357 because that is where cases begin to get sticky. To do that with Lil'Gun you will have to compress the powder charge with a case mouth expander die, else you will severely mangle the bullet trying to seat it.
As far as SAAMI max for the 38 Special vs 357 Mag, you are aware the reason for the low rating is due to the fact there exist certain marginal 38 Specials that would explode if some retard stuck a truly +P load in it? As it stands, a 18,000 PSI +P load will not harm anything out there.