I have never seen any definitive clinical studies about how and whether psychotherapy works for people who suffer from extreme hoarding.
Nevertheless, I am happy to provide some pertinent information on the topic, via a recent episode of South Park. Some of you might find it slightly cartoonish, but the episode does provide us with serious insights into one possible way that psychoanalytically-inspired psychotherapy might deal with hoarding. The link will bring up the full episode.
My thanks to Ari Mendelson for bringing this to my attention.