I updated my 2012 13" Retina MBP tonight from ML to Mavericks, and I definitely notice a huge improvement. The stuttering isn't completely gone... if you have a keen eye for catching UI lag, you'll still see it on sites like The Verge or Polygon. Even Jalopnik's GIF-heavy blog view chokes the system a bit. BUT(!), it's vastly improved over what I was getting with Mountain Lion. It used to stutter on everything - even MacRumors on occasion, or when transitioning to fullscreen. All that is gone, and it definitely feels like a new computer.
I wouldn't be surprised if early impressions are right, and the new 13" is completely lag free. I feel like my 4000 GPU is 90% there.... just a tad more power and things would be perfect. Though, for most the population, I imagine they'd be very happy with the performance I'm already getting.