I personally would try the SEB by itself and see how it goes. The amount of fiber is a trial and error thing and there are issues with too much fiber as well. Give the SEB by itself a few days and see how it goes.
I'm not sure what you mean by "improved" in terms of bulk. Cats naturally have stools that are not bulky at all. This is why when treating megacolon, a low fiber, low residue diet is most effective. Cats naturally have almost no fiber at all in their diet. They should have to "push" a little bit to pass stool, and this, with stools that give but are quite firm, keeps their anal glands healthy.I do have a concern that the SEB may not be enough bulk - Obi's stools were much improved with the metameucil...any thoughts?