I've shot a bunch of .40 Supers through my G21. I don't have any opinion about the alloy-framed Sig, but I do know that, using a threaded, compensated Accu-Match barrel and a 22# spring, I have no problems with .40 Super.
.450 SMC has exactly the same dimensions as regular .45 ACP, and can be fired from any .45 ACP barrel, if you're shooting from a good platform with appropriate springs. I'd personally want a barrel with a real, screw-on compensator, for any of the really hot new cartridges like .40 Super, .45 Super, .450 SMC, etc.
I fired several thousand rounds of .400 Cor-Bon through my G21, before having one of my Accu-Match barrels reamed to .40 Super by Aro-Tek.
I'm currently reloading .40 Super, using a convoluted system of .400 Cor-Bon dies, along with a .45 ACP Lee FCD, for post-sizing.
Over a year ago, Triton promised a bunch of us on Glocktalk a free .40 Super "starter kit", including brass, ammo, and dies. So far, I've gotten everything but the dies, and they're supposed to be coming soon. I sure do hope so, as I'm going through a lot of tedium reloading with the system that I'm currently using.
.40 Super is a really accurate, flat-shooting round. JR, of Lonewolf, sells lots of .40 Super barrels, and uses .40 Super, shot through a G21, for hunting. Last season, he killed a large cougar, which died instantly, with one shot, said shot having first passed through a four-inch limb.