But alas, your question was concerning caliber. Some fairly solid data exsists that would seem to prove that the .357 Sig (when fired from a 4" bbl) has identical terminal performance to the legendary 125 grain Magnum load. However it has some drawbacks. The Sig round is a pain to reload due to the taper crimp that was so foolishly selected by the creators of the cartridge. Also, it is pretty limited (compared to the .357 magnum) when it comes to bullet weight and style. For example, you're able to find anything from 110 grain to 180 grain bullets for the .357 Mag and don't have to worry as much about increased overall length because you're not feeding it through a magazine. Those 180 grain hunting loads are nice. The technology was a long time coming.
But the Sig load is what it is and nothing more. It was intended to simulate the 125 grain .357 Magnum load, and does it quite well. Thus it has accomplished the goal of it's designers.
Cheers.