Pediatric Physical Therapy  published a review of the literature on seating adaptations for children with cerebral palsy.  The literature revealed inconsistent results with saddle seats and the what the best seat to back angle is.  Significant improvements were seen with seat inserts, external supports and seating systems.  There was no significant evidence to support that changes in postural control improved functional abilities.   

Reference:  Chung, Julie BHK, MPT; Evans, Jessie BHK, MPT; Lee, Corinna MSc, MPT; Lee, Jessie BHSc (Hons), MPT; Rabbani, Yasha BSc, MPT; Roxborough, Lori MSc, BSR; Harris, Susan R. PT, PhD, FAPTA, FCAHS Effectiveness of Adaptive Seating on Sitting Posture and Postural Control in Children with Cerebral Palsy.   Pediatric Physical Therapy. 20(4):303-317, Winter 2008.