I was told by a Gastroenterologist that soluble fiber would help my symptoms for IBS. Would Benefiber be a good choice? The kind you mix in drinks?

Yes. Benefiber can be a good choice for obtaining more soluble fiber in the diet as an attempt to treat IBS. It is generally better to begin slowly and gradually increase the amount of fiber supplementation over the course of weeks to months. It is also important to increase your fluid intake when you increase your fiber. This works in some patients with IBS. Good luck.