I suppose if one wants a little more velocity in their 9mm it would be the way to go, but that's all it is. A slightly souped 9. A lot of rounds look good on paper, but paper is usually just that. Paper. A necked down 40 casing is not going to make a 9mm into a 357 magnum. There' a lot more to it than that.
You will get a little more penetration, BUT! You WILL NOT, get the expansion, and energy deliverance of the TRUE 357 magnum. It's all about caliber SHAPE, and bullet DESIGN, not MATCHED SPEED, i.e. velocity. Equal velocity and bullet weight, do not equal "like performance".
If Sig had named the cartridge the 9mm Super, it would have still been sitting on the shelf. Instead it "PLAYED ON" the legendary 357 magnum's excellent reputation as a police fight stopper, and PAID GUN WRITERS exploited the readers in INVENTING THE 357Sig A GOOD REPUTATION. And that is what they did. They "invented" it along with PAPER THEORY logic.