aka: elp_jc quickly squashed my thoughts and said that it's is NOT in the throttle sleeve.
Know it all (that's MOB, by the way)? And playing the woman victim here? Geez; thought you were better than that. Anyway, I simply stated that on a bike with a POSITIVE THROTTLE CLOSING cable working and adjusted properly, if you can close the throttle,
binding or not, you close the throttle blades (no air), and no high-rpm rev-hang should occur on a stock bike not designed to do that (some are via a solenoid on the throttle stop). In addition, if throttle is not binding at all when stationary, and with a HOT engine, it's unlikely the problem is with the plastic throttle tube. But it is possible. Remember the guy hasn't found the problem, so at least wait until that happens if you want to play your (fully unwarranted) BS woman victim card.
Now, with the 2 new revelations (a tune and 'metal to metal' contact), he could have TWO problems. If the ECU keeps injecting fuel for a moment with the throttle closed, and the throttle closing cable has some play in it, revhang could occur for a second or so. And the idle air passage alone could allow that even if the throttle plates are closing properly. Therefore, I wouldn't rule out the tune at all. But the culprit could be the tune or a bad TPS (throttle position sensor), so the only way to eliminate it as the rev-hang problem is to put the ECU back to stock and see what happens. Next, the 'metal to metal' contact is a clear sign of a problem somewhere. Therefore, if I was the OP, I'd just flash the ECU back to normal, and if the problem persists (ruling out the tune), take it in under warranty and let a dealer/Kawasaki sort this very weird problem you're having. Good luck.