I don't know if the pumpkin slows down digestion. I don't think it does digest, that's the nature of fiber, right, it doesn't digest, it just helps push things through? I have two cats on it now, more for constipation than for hairballs, but now that they are both on it daily, I have stopped using any hairball remedy either, and it seems to be doing double duty.
When I started my first girl (not struvite kitty) on it, I made no other changes in her diet. She's never had hair ball trouble anyway, just a lot of trouble passing stool, which is
why I started her on the pumpkin. Almost immediately, her constipation issues disappeared. Within six weeks, she had slimmed down, and become at least twice as active as she has EVER been. She lost half a pound, but it really is more that she's become fitter (fit and trim, I call her) from the increased exercise, and not carrying around a lot of sludge in her colon.
My struvite girl is still having a bit of constipation issue, even on the pumpkin, but I think that is from the pepcid. SHE was the cat who has had hair ball trouble from the time she was about one year old. Her first hairball cost me $400 in emergency vet fees and x rays. From that time on, she has had to have a regular dosing schedule of hairball remedy. Every three days from November to February, every other day from February through October. And that was BEFORE she was on the c/d.
The constipation problems started in November 2010. She was still on the c/d then, and even though she was getting so much hairball gunk, she was still having troubles. I was giving her psyllium mixed in a watery canned meal every four days or so, even though I KNOW psyllium is NOT good for cats, and can only make the problem worse, I just didn't know what else to do because she wouldn't, then eat the pumpkin, and it did seem to give her some relief. Then, this past summer the regurgitation started, and I struggled with that until I finally made the switch off the c/d all together, which you know about and have been a big help with (as have all my friends here).
She still refused the pumpkin, until I tried just giving her really tiny amounts in her meals, to help her get used to the taste. Gradually I was able to increase the amounts, and now she accepts a half a tablespoon twice a day in a meal. So then, I stopped ALL hair ball remedy for her (though it was a very scary thing to do)
Since I have stopped the hair ball remedy, she has had no hair ball problems, I am positive the pumpkin is taking care of it, helping her pass the fur in her stool. She also is not regurgitating, and her digestion seems to be doing fine. She is, however only having a bowel movement every 48 hours now, and that with mineral oil help but I think that is from the pepcid, not hair balls or trouble with her digestion. She is still on the probiotic.
Sorry, I didn't mean to make this thread about my girl and her problems, but I didn't know how to explain my experience without bringing her into it.
I think, in your case, I would mix the pumpkin with canned food, and continue the raw treat alone, as you have been doing.
Geesh, I need to learn more about how various dietary fibers work, I think. I read an article about the various kinds once, but don't remember where. I know that pumpkin fiber is NOT a bulk forming fiber, so it doesn't put our cats at risk of megacolon, cause dehydration, and make constipation worse (like psyllium can). I am pretty sure it does not interfere with nutrient absorption like laxatives do (mineral oil, lactulose, hair ball remedies), but I don't remember WHY I know that, I researched pumpkin when I first started it with my first girlie, but that was a while ago now I can't remember what my resources were.