He would simply be my first pick in any T20 team. Maybe not the highest SR, but his SR is still above a run-a-ball and he averages 40 or so. This makes him a great guarantee for a good start of the Innings. Maybe other players are just a bit better than Kallis, but non of them come even close with the ball.
BTW, Kallis is still in the top list of players who 've hit the most sixes at the WC T20.