Something needs to be understood here. Velocity has no bearing at all on the "+P" designation. +P simply indicates that the loading develops chamber pressures above the SAAMI specs for the caliber. It is possible to have a standard velocity load that is high in chamber pressure, just as it's possible to have a higher velocity std pressure loading. Velocity and pressure has a sort of interdependence, but they are not synonymous or interchangeable by any measure.

Now, back to the original post. My experience has been that 38 Spl ammo has remained pretty consistent pressurewise over the years. However I have noticed downrating of the 357 magnum pressures over the last few decades. I've also heard complaints that the 357 loads are down to where 38 Spl +P performance used to be. Loading data from the sixties vs todays data seems to support these claims, however the powder manufacturers have claimed that they simply have better ways to measure chamber pressure, and that only current data should be used. That said, my old 38 +P load is wayyyy off the charts nowdays, and the old Model 36 I used to shoot it in never seemed to care about it at all.