How well the robot will turn is also based on the placement of c.g., as described in this paper:
http://www.chiefdelphi.com/media/papers/1443
If anything, the zero-radius turn will be somewhat off-center because there is no symmetric place the center of gravity can exist while also maintaining stability for the other turning configurations AND being far enough offset from the center of area to reduce the torque/current needed. Off-centered-ness isn't a big deal, but it may muck with autonomous mode if zero-radius turns are heavily depended upon.