im using 15 mainspring but you should at least have a koeinig set, limcat extended firing pin and titanium struts and caps for it to be reliable.
i also have a lightened slide and tungsten guide rod. i liked 12.5 ismi recoil spring till i tried an 11 pound rs ismi. I may soon go 10 pounds.
Rule of thumb for ipsc shooters is the lightest springs you can go with 100% reliability