I finally found the right deal on a Sig I've always wanted. It's a P226ST .40S&W. I already picked up a .357 barrel, and MK 25 slide assembly to make it multi caliber, as well as Grayguns upgrade parts.

My intent is to use it for IDPA and USPSA competition. My question is about IDPA legality of the P-SAIT trigger in SSP. The rules say no external modifications, BUT they also say you can use parts from a different model by the same manufacturer if they are drop-in. Since Sig used the P-SAIT on the Legion series of pistols, I'm interpreting that it's therefore allowed for SSP. Am I right on this, or am I pushing my luck?

In my opinion you are correct.

I think it's likely that no one will know the difference if your gun is inspected unless you tell them. You could always try emailing your regional coordinator or IDPA HQ. For a local/club match it probably doesn't matter at all unless you just want to attempt to follow the rulebook to the letter.