Colt puts 14lb. springs in their 9mm and .38 Super pistols, while Springfield Armory puts 9lb. springs in their 9mm 1911s. A lot of people bitch about the Springfields not working well until they swap springs to heavier weights. That might tell you something..............
I bought a new SA Range Officer 9mm about 3 years ago, and had some issues, so I chucked in a 12lb. spring, and those issues went away. Immediately, gone. When I rebarrelled that RO to .38 Super, I did put in a 14lb spring, if nothing else, to keep it from slinging brass into another county. It worked fine.
Another tip, use 124gr. ammo for the first thousand rounds or so, the weak-kneed 115s barely have enough balls to cycle a 14lb. spring. You'd probably get better accuracy, too.
I have not done a 10mm to 9mm conversion, so I don't know if you'll need another slide. extractor, ejector, etc. You'll have to let us know about such things.
