I was cruising around on sarcos sight and they have a 9mm conversion kit and it says it includes the correct main spring for 9mm. Why would it matter? A primer is a primer, they all need a certain amount of impact to go off.
Also I think it was on sarco I saw another mainspring that said it was for aluminum frames. Is this something to worry about? I already built a 1911 on a stealth arms aluminum frame with a regular mainspring.

Edit: I'll just go sit in the corner. 

9mm has less recoil, so the slide might not have enough energy to cock the hammer when a stiffer mainspring is installed.

For someone who claims to have built a 1911 pistol your lack of understanding the operation is alarming.

At least 90% of the slide energy is controlled by the mainspring acting against the hammer and firing pin stop. This is why the selection of mainspring weight is critical, not primer impact.

Stock 45 mainspring is 22 - 23 pounds, IIRC .  9mm, 38 super, and Gold Cup 45 main springs are 18 - 19 pounds IIRC.

So you can apparently use the "weaker" spring in a 45, but a 9mm might not completely cycle with a 45 mainspring.