I think an even better question is how much less suffering would there be in this world without theism?
If you get cancer and suffer from it that is awful, we are conditioned that it needs to be attached to something specific like God is causing it, or God is allowing it, or the Devil did it. Just imagine for a moment its none of those things. Then its just a bad roll of the dice.
Now there is suffering inflicted upon people and animals.
I think it's not a sweeping solution, but I believe in continuous improvement. If you have someone that hurts animals just for the sake of causing suffering, or people, like a child molester, they are a bad force in themselves and should be dealt with accordingly.
Ideas can also be bad, such as islam, watch the Ben Affleck and Bill Maher argument. The ideas of it and its laws lead to a lot of human suffering, its a bad idea, it should be eliminated. Who knows over time how much "evil" or "suffering" could be eliminated with actually working on removing the true causes and not just saying it's and unknown evil force at play, like god and satan wagering over the suffering of Job. Thats my mom's mentality, I have suffered my life the devil did it, the devil did it, the devil did it. Everything. Persecution complex.
I like this quote, the idea behind it is very good. Even though I am an atheist, I would have no problem quoting a teaching of Jesus if I feel it made sense, so likewise I am not advocating any religious group here, but the quote is good:
“When you think everything is someone else’s fault, you will suffer a lot. When you realize that everything springs only from yourself, you will learn both peace and joy.”
- His Holiness the 14th Dalai Lama