That's good that "The Lump" is improving. I'll tell ya what has healed many cats is an all raw meat diet. When Cubby adopted me he had TERRIBLE diarrhea and raw meat was the ONLY thing that helped it. Dry or canned cat food [even top quality organic] only made it worse but raw meat eventually got him over it, and it took several weeks. I think the stress of the move here also aggravated it.
Luke had runny, terrible smelling stools on dry cat foods. On canned they got better, and on raw meats they got better yet.