A .357 Sig is, if you use top ammo, a true .357 Magnum of 1930s vintage.
Possibly, if fired from a 4" barrel and restricted to only the 125gr (or lighter) bullets.
The .357 Sig was created to do one thing, replicate the .357 Magnum 125gr load fired in a 4" barrel revolver, in a 9mm size semi auto.
That .357 load was touted as the "king" of street stops, and having that level of performance in a 9mm size semi was felt to be a good thing.
The .357 Sig does not, and CANNOT match the .357 Magnum in any other load, bullet weight or barrel length.
Do not blindly accept statements about the .357 Sig being the equal of the .357 Mag in general. And don't get too hung up on a handful of FPS, either.
Statements of how they are similar bullet weights at similar speed are meaningless unless you have a definition of "similar" specifically spelled out.
Variations of as much as 100fps are not unheard of, in similar guns with similar barrel lengths, firing the
same ammunition. Usually the variation is much less, but there is variation.
Also I am curious, since I don't buy any, but just how "cheap" is that 9mm+P+ ammo???
I have several 9mm autos, none of them rated for +p, let alone +p+, so I don't buy it. When I want more than a standard 9mm, I go to a larger round.
I'm pretty confident my Contender will handle 9mm+P+, but that's not an auto pistol.