I have a STI Trojan 9mm 5 inch. I think it came with a 14 pound recoil spring. The 14 seemed too heavy to me, so I changed it out for a 12.5 pound spring. The 12.5 has worked great with all factory and my reloads.
Some of the guys I shoot swear by 9 or 10 pound recoil springs. I think these would be a tad lite for +P ammunition.
I have never changed the main spring on this gun, but my guess is it is
about a 19 pounder. It has a great trigger and I have never had a round not fire.
As a side note, I run 19 pound main springs in my other 1911 game guns.
I put a 14# spring in my Springfield Loaded 9mm (5" barrel) but it didn't work with standard 115gr practice ammo (like Winchester white box and Remington UMC). I was getting stovepipes every single time... oversprung...
Then I replaced it with a 10# spring and it's running fine. Maybe a 12# spring would have worked, I haven't tried it.
The mainspring is 21#.
Every gun locks up a bit differently, so I always try to use a spring combination that will throw the brass from 3 - 6 feet from the gun. For standard loads, I use a 19# mainspring and a 12# recoil spring. For +P loads, I up the recoil spring to a 14#. You many vary this with your gun by determining how far the brass ejects. If your +P load is really ejecting them over 6 ft., you may want to up the mainspring to 21# and keep the recoil spring at 14# or possibly 12#. I usually get better sight tracking under recoil with a lighter recoil spring that ejects the case about 6 ft out.