For adult rapists, I find the following data from one review article* that looked at 10,998 subjects covering many categories of sexual offenders:
Recidivism rate after treatment: 20.1% (79/393 perpetrators)
Treatment included relapse prevention, and behavioral and group therapy.
Recidivism rate without any treatment: 23.7% (32/135 perpetrators)
Within this category (adult rapists), recidivism rates appear comparable w/ and w/out treatment. However, looking at the data for some juveniles and for non-violent sexual crimes (such as exhibitionism) it appears that intervention therapy has a more positive effect in those cases.
*M.A. Alexander, Sexual Offender Treatment Efficacy Revisited, Sexual Abuse: A Journal of Research and Treatment, Vol. 11, No.2, 1999.