As Jim Watson points out, some loading data for the .40 S&W has been tamed down, due to unsupported chambers in some firearms.
The 357 Sig isn't just a .40 S&W case necked down. The case was actually engineered from the ground up to contain higher pressure than the .40 S&W. It has a thicker web and lower wall area than the .40 S&W case does. It's also a longer case.
I shoot both calibers, but have to admit that I shoot very little .40 S&W as compared to the other calibers I shoot.
I would equate your question to a similar one about the .38 Super and 9x23 Winchester. Both cases are basically the same on the exterior, but differ greatly on the inside. The pressures the two rounds work at are very, very different, and the cases are engineered to handle the working pressure of each caliber. It's all in the case.
Hope this helps.
Fred