As listed in the IDPA rule book, your ammo needs a power rating of 125,000. Any bullet weight above 125 gr can easily acheive this with not much danger of fouling. The problem is I wasn't able to find any load date for lead bullets in my Speer, Hornady, or Lyman manual. According to Lyman ".. the short neck of the 357SIG proved unsuitable for use with any of Lymans cast bullet molds."
According to all the specs I was able to find, the SIG is sized at .355. A cast .380/9mm round is sized at .356, which should work in the SIG. Friendswood Bullet makes a cast 147 gr 9mm bullet which at 850 FPS or greater will make you IDPA legal.
Will the Friendswood bullet work in the SIG? I don't know. There still may be a case length issue. Then again, just because the Lyman bullets won't work doesn't mean nobody else's will. I load cast bullets in .380 ACP all the time, and they're quite short.
Perhaps you can find a bullet caster that will sell you 100 to try out, or better yet, maybe they'll have a definate answer.
Good luck finding a solution to your dilemma.
GH1