I'm not sure what you are feeding your cat. I can tell you when we got the diabetic diagnosis, we switched to hills md and purina dm per vet instructions back in november and december, i began noticing mr biddles making straining noises in the box when pooping. Changed vets, they said try hills jd mixed in with hills md. This made him poop like crazy but not in a good way. The hills jd gave him gas. He's never had gas before. But at least no grunting in the box then. It did also however send up his glucose, which really stank and only contributed to prolonging his illness.
As a side note, it also became apparent at that time of some hind leg weakness and possible arthritis, so this could have contributed to difficulty pushing out poop, and/or discomfort with that position. Peeing was no trouble, which was good, as he was peeing a lot!
On my own i tried powdered psyllium husk, powdered pumpkin (since mr b would never eat it from a can), and miralax, all which seemed to help but never kept him regular. Also was adding fish oil (nordic naturals) which also helped a little. The powdered pumpkin, if you are interested, is called 'firm up!' by 'diggin your dog'. Its for cats as well. I bought mine on amazon. I've added link below.
Now i cant say for sure since it is only day #2 of his new food Ziwipeak but his last 2 poops have been excellent. That is all he is eating, no laxative supplements. We'll see.