I am a horrible shoulder sloucher. Arching my back would not do a thing. I'd look like a shrimp. If you're an overall hunchback, then yes, arching would even that out. It would be more a muscle memory relearning thing. If your back is flat all the way up to your shoulders and goes bad from there, overcorrect for awhile by bringing your shoulders back with the intent of trying to touch your shoulderblades together. Over time it will also even itself out in terms of muscle memory. A good way of figuring out which problem you have is to ride with a long crop sticking out of your pants. If you hit the crop lower down, you're a hunchback. If you have contact with it all the way up to your shoulders and then lose it, you're just a sloucher. (no insult intended!) Either way, it's fixable.
You just have to see your distance...you don't have to like it.