http://www.sigmatrigger.com/ has a video that shows what's going on with the spring. the Apex kit doesn't mess with this, it's just Wolff striker spring, etc. if you do any of this, never admit it, even under oath, you know nothing!
I shoot with an SD9VE owner and we trade guns for a few magazines, he likes my Kahr and I like his S&W. I kid him and ask if he brought his Glock, hehehehe.
(some gun co.'s will sue another co. for stealing designs, see Glock vs S&W or Kahr vs Diamondback. other co.'s will charge for being copied, see Kel-Tec/ Ruger (P3at/LCP) or Colt/ Sig (P238/Mustang).
long story short, the Sigma or SD can be made very Glocky for cheap. good shooters. the lighter you make the trigger, the more apt you are to Glock yourself---
http://rt.com/usa/indiana-police-chief-councellor-981/ . good luck.