Is it worth it to conquer backwards states so they can benefit from your technology? I think a conquered Peru, for example, would produce more iron and sulfer than a native controlled one, even if the workers receive less income. If it is a resource you need / that is valuable, shouldn't taking the province from a low tech nation boost it's output?
It should be increasing the output, but it won't make your richer unless you're selling them more industrial goods.
Because their POPs also demand more goods with tech advancement. (raising demands for goods a lot)