he's strictly worse than Coercive Portal
I'm not even sure that's true.
On T0, Karn has drawn you the worst of two cards. Portal has done nothing.
On Tn, where n is odd, Karn has, in the worst case, drawn you the top n+1 cards of your library. That's if you alternate his plus and minus ability. Portal has drawn you the top n cards.
In the best case, Karn has dug you deep into your library and drawn you the best of these cards.