I'm not taking anything away from the 357 Sig.Its SAAMI max pressure is 40,000 psi. Standard 9mm and 40 S+W are 35,000,9mm +p is 38,500.
10mm Auto is 37,500.
If a gun is designed for 10 mm auto,a .357 Sig would be at +P pressure for the gun.
My understanding is that the Sheild was designed for the 40 S+W from the start. Both the 40 and the 9mm standard is 35,000 psi. All other things equal,including pressure,a larger cartridge with greater bore/breech surface area will stress a gun more,as the pressure is applied to the surface area.
In a Shield,the locking surfaces and breech will have roughly the same experience from a 9mm +P as they would from a 40 S+W. I'd expect a Shield in 40 S+W to last about the same round count as a Shield fed 9mm+p. However,thats just my opinion.S+W might disagree.
If the 357 Sig is in a particular gun,say a 1911,brass strength aside,the 40,000 psi 357 Sig is just as hard on the gun as 40,000 psi +p+ 38 Super loads.
There is no free lunch.Everything is a tradeoff.