i took a razor blade down the sides of some rootbound mommas i had before i transplanted them, they are looking great now (a month later).
i've root pruned mommas before by destroying the rootball down to nearly nothing before putting them back in the same container with fresh soil, they always grow back nicer.
I take my transplants and chop the sides with a shovel.. a proper spade and chop an X into the bottom so there are 4 pieces and spread them before putting them into the ground.