Honestly, after dealing with IBS for 20 years, I don't believe stress plays that big a part. While a seriously nerve wracking experience (speaking in public, flying on a plane, etc) might cause a sudden change in symptoms, it does not impact the overall disorder.
I was in psychotherapy for nearly 22 years. I have effectively eliminated all external stress from my life. I have a great job, great kids, strong faith, good friends, etc. I watch my diet, don't drink, take no drugs, etc. I'm generally happy and joyful and peaceful. I still suffer everyday with IBS pain. My gut is the CAUSE of any stress I have in my life.