What you must understand is how kinetic energy causes damage to the human body. Basically kinetic energy transfer is almost directly proportional to the size of temporary stretch cavitation. Temporary cavity injures by stretching tissue beyond its elastic limit. Many discount kinetic energy alltogether because they believe that handguns do not have sufficient energy to produce a large enough temporary cavity to cause permanent damage. This isn't entirely true because the tissues of the body are not homogenous. Certain tissues like the liver, spleen, small blood vessels, and certain nervous tissue are quite delicate and can be easily damaged by temporary cavity produced by handguns while other tissue like muscle, bone, and lung tissue are not so easily damaged and cannot be reliably injured by temporary cavity.
With regards to the magnum-type calibers line 10mm, .41 Magnum, and .44 Magnum being overly penetrative, remember Newton's third law: every action has an equal and opposite reaction. The more energy with which a bullet strikes its target, the more energy it must expend in order to continue to penetrate that target. While a .44 Magnum may exit a target and thusly not expend all of its energy, it may have still expended just as much as a .357 Magnum because the .44 had a lot more to begin with. A good example of this is the gelatin test of Speer's 240grn Gold Dot over at Brassfetcher.com. While the bullet did go completely through the 16" gelatin block (just like the 125grn Gold Dot .357 Magnum did), the temporary cavitation was so great that it actually broke the wooden board that the gelatin block was sitting on. The biggest drawback to large calibers like 10mm, .44 Magnum, and .41 Magnum is, IMHO, their increased recoil and the size of the gun necessary to shoot them.
Coming back to the .357 Sig, you must understand that it does not match what the .357 Magnum is capable of. The Sig is really sort of a one-trick-pony in that it can match, in certain loadings, the performance of the older 125grn SJHP .357 Magnum loadings. However, the Sig cannot match the velocities attainable with hotter .357 Magnum loadings from firms like Double Tap, Buffalo Bore, or Grizzly, it cannot match the performance of .357 Magnum loadings with heavier 140-180grn bullets, only a handfull of Sig loadings match the velocities of older 125grn .357 Magnum loadings, and the bullets used in the Sig don't always perform the same way as those used in the Magnum. In order to understand the reasons for this, you must understand the history of the .357 Sig cartridge.
The Sig was originally designed to duplicate the performance of the 125grn .357 Magnum loadings in a semi-automatic platform. The original loadings of the cartridge were able to match the ballistics (125grn @ 1450fps) but they unfortunately suffered from projectile dysfunction. You see, the .357 Sig uses a .355" diameter bullet like a 9mm or .380 rather than a .357-.359" bullet like the .357 Magnum and .38 Special do. The original .357 Sig loadings were using bullets that had been designed for 9mm velocities and, unsuprisingly, they displayed over-expansion, fragmentation, and underpenetration when driven at such high velocities. The solution to this was two-fold. First, the velocities of most .357 Sig loadings was reduced from 1450fps to roughly 1350fps where most manufacturers keep it today. Secondly, newer, cartridge-specific bullets were designed that would better hold together at increased velocities.
The Sig, in its hottest loadings, still is only able to duplicate the velocities of run-of-the-mill Magnums. Custom ammo companies like Double Tap and Buffalo Bore are able to drive 125grn bullets in excess of 1600fps in the Magnum, velocities well beyond what the Sig is capable of. Likewise, the vast majority of Sig loadings use 125grn or lighter bullets. While there are a couple of 147grn .357 Sig loadings (Hornady and Double Tap IIRC), they are unable to match the velocities of similar or even slightly heavier bullets in Magnum loadings from the same manufacturer. Also, the Sig's heaviest bullets are 147grn, a weight that is merely midrange for the Magnum (JHP .357 Magnum loadings are available all the way up to 180grn and hardcast 200grn loadings are available).
Finally, remember that the best .357 Sig loadings are matching the performance of .357 Magnum loadings that are 30+ years old. Newer Magnum loadings like Speer's 125grn Gold Dot are able to display excellent expansion along with much deeper penetration (this loading penetrated 16"+ in Brassfetcher's tests). The advantage to enhanced penetration is that the temporary cavity remains larger in diameter deeper into the target thusly giving it greater potential to injure vital organs that lie deeper in the body.
Basically, what the .357 Sig gives you is penetration and expansion very similar to a 124-127grn 9mm +P or +P+ with a slightly larger temporary cavity. Due to its higher velocity, the .357 Sig also has slightly better barrier-penetration capability than either the 9mm or .40 S&W and significantly better than the .45 ACP (.45 has never been known to fare particularly well against intermediate barriers other than auto glass). I'd consider it to be in the same category as other common service-type calibers such as .38 Special +P, 9mm, .40 S&W, and .45 ACP.
