I don't have any experience with their Synergy line, but I have a friend who had tried all sorts of foods for her greyhound with IBD-type symptoms. Her vet recommended the Natural Balance Limited Ingredient foods to her. All her issues have now cleared up -- no more diarrhea, soft stool, gastric distress, etc. I can't remember which one, but she's either on the bison or the venison now.
Shortly after her positive experience I tried it for one of mine who had developed horrible dry, itchy skin and inflamed ears. The Limited Ingredient Fish and Potato did the trick. She seems to have developed sensitivity to chicken, turkey, and to a lesser extent duck. If she eats any of those protein sources or foods with grains in them and she starts scratching like mad and gets an ear infection.
It can be a long and frustrating road to finding the right food. My friend's vet recommended the Limited Ingredient line to help narrow down more quickly potential problematic ingredients. The Synergy line is a multiple protein source food (chicken and lamb) and multiple grain (rice, barley, oatmeal). Your dog's issue may not have anything to do with grain or protein sensitivity and may respond well to the additional fiber alone, but it's something to be aware of if you've tried a lot of different foods with no success.
Good luck with whatever you decide to try. It's a beautiful thing when find the right food.
ETA: I am now feeding my grey TOTW Pacific Stream and it works great for her. The Limited ingredient trial helped me figure out which ingredients were causing problems for her.