Fosty, I am 45 and have had my j-pouch for 8 years now. I think my pouch has only gotten better over the years rather than the other way around. Part of that is understanding things and what works for me. The addition of sulfasalazine and Prilosec has done wonders for my frequency and acidity, reducing the # of antibiotic rounds I take for pouch issues a lot. So to that question - I can see where things might regress, but that hasn't been my experience. Doesn't mean it couldn't / won't happen.
If you know you have a stricture (for whatever reason), yes - foods definitely have an impact. Anything fibrous that is difficult to chew could cause partial or full obstructions. I find that fiber itself (Metamucil wafers) didn't cause me problems when I was struggling with obstructions, but there were definitely foods I stayed away from. I had adhesion-related obstructions in the small bowel that my surgeon released (surgically). Also, grape-juice of any kind is what I call "Liquid Plumber for the Gut." If in doubt, during or after eating difficult foods, have a glass of grape juice too.
Steve