The moment of "surprise and delight" when Kirk gets out his communicator and says to Spock "It's two hours - are you ready?" would surely have been ruined if in the earlier scene, Kirk had followed his "KHAAAN" outburst by turning to the others and saying "That should hold him for a while." So as a result we never get to find out whether this was part of Kirk's plan or not; to me it's a worthwhile tradeoff.
I suddenly have a vision of Kirk doing this without switching off the comm channel, a la Galaxy Quest
Khan: "Perhaps it will not hold me for as long as you think?!"