Has your kitty been dx'd with Megacolon?
First of all I would do the surgery last resort. And I would get your kitty off of the added fibre. This is the old way of thinking. And it sounds like it is not working for your kitty either. What medications and doses are you giving your kitty. A great stool softner is Miralax (called PEG 3350 in Canada). And if the stool softner and reduced fibre diet (MAX 1 to 1.5% Fibre content grain free canned food) does not work alone then your kitty may need a motility drug called Cisapride. My kitty has Megacolon and has been doing well on tis regimen. And yes at first the vet wanted me to do the high fibre, lactulose route too but they did not work.
You may want to join this group that deals with constipation and megacolon only
http://groups.yahoo.com/group/Feline_Megacolon/
Lots of great support and advice for you there. And many people that have had the surgery done on their kitties.