I've been loading the .357 SIG for four years now. I use Redding TC dies and a Redding profile crimp die in the fourth station of a dillon RL450 B(fore runner of the 550). I use Hornady 125 gr FMC for practice, and Speer 124 gr Golddots for carry loads. Best powders have been AA#7 and Alliant Power Pistol. The Power Pistol has given loads of 1410fps over my crono at 5'. This is from a 4" Glock Model32.One thing of note is that the .357 SIG headspaces on the casemouth not the shoulder as you might expect(I have no clue why they did this). Therefore a taper crimp is called for. Also don't try to make .357 SIG cases from the .40S&W, because the SIG is .020 longer than the .40 S&W. Using the Redding dies I've had no problem with bullet pull at any time.
I have loaded 357 Sig in the past, but abandoned it because it is a pain. I did have alot of fun with 90 grain bullets going very very fast I used both RCBS and hornady dies and found that the cartrige headspaced on the shoulder, and not the case mouth, with once-fired brass anyway. I also learned that 9mm bullets vary in diameter. Noslers, for example, measure .354" and Speers .356". This is important as far as bullet setback is concerned because the expander plug is fixed in diameter. Noslers suffered alot of setback. Speers suffered no setback. I also found that nickle-plated cases held-on to the bullets much better than plain brass. I used .40 S&W brass to make 357 Sig cases, but abandoned that practice because I found the longer 357 neck more desirable in terms of neck tension.
90gr. Gold Dots at 1800 FPS from a 6" KKM Glock 24 barrel were a blast. What a useless load, though.