The belief in a god is a symptom of a more rudimentary problem - a staggering lack of critical thinking and evidence-based investigation. While most theists manage to compartmentalize their theistic beliefs from most of their day-to-day decision making, the same ineptitude towards applying rational, logical thought patterns to other decisions in their lives is more likely to hamper their capacity to make wise ones. It's "faith based thinking" - superstitious thinking that one has to practice skepticism and critical thinking to override. If you're out of practice/habit in applying that on a regular basis, you're more likely to believe in a god, and more likely be making other poor decisions because you aren't regularly applying skepticism/critical thinking.
The world would be a much better place if people were rational, skeptical and critical thinkers. That would be mutually exclusive with a theistic world.
This is a concept I struggle to convey in a coherent and succinct way. Does it harm you to believe in God? What I'm addressing above isn't so much that the belief in God hurts you, as much as the common denominator that leads a person to theism.
I believe I have a decent analogy to explain it. Maybe. We'll give it a whirl. This is going to be mostly suppositional, with basically no research. Just take it as an explanation of my perception of the problem.
When it comes to making decisions about what to eat, we have a spectrum of people.
On one end of the spectrum, we have those who don't really think about what they eat. They'll go to the grocery store, see that they have some boxes of General Tso's chicken, buy a bunch, and then proceed to stuff their faces without regard to calories, health, nutrients, costs... nothing. The gut feeling is the dictator of actions.
On the other end of the spectrum, we have those who crunch the numbers. They calculate calories and nutritional requirements. They educate themselves on what the different ingredients do - and decide to avoid things like high fructose corn syrup. These people may have the cravings for General Tso's chicken, but that's overridden by a higher-level cognition and application of critical thinking skills. They make better, more wise decisions based on evidence and data, than emotion and intuition. This approach is often much more difficult to maintain and execute successfully.
Further, of these people on either end of the spectrum, it's not likely that the approach taken by the two will only be applied to their dietary habits. This is how they approach most of their lives. The dietary habits are a symptom of the individual's mental habits, and one would probably find that they are making decisions about other aspects of their lives under similar protocols (or lack thereof).
The fact that America has a huge (pun) obesity problem is a reflection that America, by and far, has a set of standards and values that does not include making good wise intellectual decisions about eating and exercise.
Likewise, the fact that America has a significant amount of theism is a reflection that America, by and far, has a set of standards and values that does not include proper epistemological and evidentiary educations, let alone their application.
We can only hope that the non-application of skepticism, critical thinking and evidence-based investigation is compartmentalized to only this one topic - the existence of a god, within these people.
That, however, is not likely.
The amount of harm from poor decisions made due to their ineptitude of epistemological and evidentiary reasoning, in the remaining aspects of their lives, may be truly incalculable.
I'd mention that what I'm not doing here is merely equivocating between fat people and theists. I consider myself a reasonably good skeptic and critical thinker. I am also overweight, eat badly, and cause gravitational lensing wherever I go. The complexities of psychology can create situations where other factors can overpower your thinking skills. I've tried to lose weight on many occasions. I know how to do it. I just can't keep myself on the right path, due to other factors.
Likewise, many theists may have attempted applying critical thinking, etc, but found they were overridden by other aspects of their lives too. This isn't a slight against theists. Nobody is perfect. It's also why I disregard "deathbed conversions" - a dying person is terrified. Reason and logic is not bound to be top on their lists of things to do.
Would the world be better without theism? Yes, and no, as I've explained. Will we accomplish that? Probably not. With better education on critical thinking and skepticism, I think we could improve our species quite a bit, though.