I specialize in proofs of King's books. I have 122 US and 46 UK proofs, plus assorted manuscripts, hardcover advance reading copies, etc. Most of them have no changes with what is actually published, but occasionally there are some minor changes. Regarding CD's unbound proofs, I haven't really compared them to what was actually published. I doubt there were any substantial differences, though.
I do agree that the unbound proof of SOD Volume 1 seems a bit more scarce than some of the others, but they have appeared so recently that it's hard to make a determination of just how many of each book were distributed. Also, you have to remember that a large number of these went to SK collectors, who will probably hold on to them.
I'm still looking for the unbound proof of Doctor Sleep, which is the only CD unbound proof I'm missing.
John