Not to say that I like the Owned proposal (especially the Deref part), but with your IntoOwned<T> you can't have these two implementations at the same time:
impl<T: Borrow<T>> IntoOwned<T> for T {}
impl<T: ToOwned> IntoOwned<<T as ToOwned>::Owned> for &T {}
The problem is that nothing prevents a U: ToOwned<Owned = &'static U> from existing.
The first impl with T = &'static U would implement IntoOwned<&'static U> for &'static U.
The second impl with T = U would also implement IntoOwned<&'static U> for &'static U.