As for why Sig went with .355 barrels instead of .357, I think someone earlier hit it right on the head -- commonality with barrels that they already produced.
Somehow, I sincerely doubt that that extra .002 to .003 diameter for the .357 bullets has any effect at all on bullet performance.
"from a loading/reloading standpoint most of the 125gr .357 bullets have a fair amount exposed lead"
That is but one bullet design for the .357 Magnum. There are many other 125-gr. .357 bullets that don't look like the Remington hollowpoint rounds.
"The gift which I am sending you is called a dog, and is in fact the most precious and valuable possession of mankind" -Theodorus Gaza
Baby Jesus cries when the fat redneck doesn't have military-grade firepower.