If iTunes got re-wrote in Cocoa, then it would be a big thing (not big as in a stand alone reason for iTunes 8, but big as in a good step forward to making iTunes more efficient and slimmed down) and it would definitely get a mention.
Maybe it would get a mention, I'll guess I'm wrong about that, but it's still irrelevant to the vast majority of people who use iTunes. Most people care if their computer and the apps on it work, not how.
As for the "more efficient"... Like I said, itunes + cocoa = itunes. Probably all software could stand a re-write after a few years and might be more efficient for it, at the very least we'd be trading old assumptions, bugs, and limits for new ones. But there is nothing magical and wonderful about one programming framework over another.
Let me put it another way, if re-writing apps in Cocoa mattered that much then wouldn't it have been done already, especially for things like FCP? As it happens, I think you might be right in the end. Not because itunes in cocoa would be somehow "more efficient" but rather because it's clear that apple are slowly pulling the plug on carbon.