I never met a donut I didn't like, but I'm not a huge fan of KK. However, "Southern Maid" or "Shipley's Donuts" are at the top of the pack around here as far as I'm concerned.
I'm sure the pig food isn't made purely out of old donuts, just a certain percentage would be part of the ration. It certainly would be a caloric enhancer; they'd get the protein from other sources. The feed mill procurement people are always looking for and substituting available low-cost ingredients to meet their nutritional targets for some particular ration. If the stale donut deal fell through, they'd switch to something else similar. I think feeding pigs stale fruitcake could be considered cruelty to animals.