I was reading your post from yesterday, and you said something about asking for permission to change your cat's diet? What I feed my IBD kitty is not something that I talk much to my vet about. She asks, and I tell her, but that's it. She also has an IBD kitty, so we do exchange notes. She doesn't feed Z/D either. She calls it "hydrolized chicken feathers". LOL. Needless to say, that clinic doesn't sell it. She, and I, feed commercially available food. Z/D is horribly high in carb, and changing food will make a huge difference in dose needed.
You've been on this dose long enough with SLGS, that you could try an increase to 3.25 units now. That is, as long as you are continuing to feed Z/D.