Pigs age about 5 times faster than humans. Wouldn't pig organs age 5 times faster than humans also? That might be one more thing you needed to tinker in pig DNA. Make pigs (or at least the target organs) long lived.
All this tinkering begs the question: Which is easier, tinker pig DNA to make pigs compatible with humans, or tinker human DNA to make humans with pig-like-brains that grow fast enough to be used as organ source? Does it ethically make any difference?
What if we could grow human bodies with no consciousness what so ever (this might not be even super hard, or require DNA tinkering, just damage the fetus where appropriate brain parts are about to develop), wouldn't that be the most humane option? Since we wouldn't be killing anything, but hunk of human meat that never had a conscious thought...