there is evidence out there, especially for the treatment of children with cystic fibrosis, mainly evidence in the form of 'conventional physio' versus 'no physio' or one physio technique versus other techniques (check out: Main et al 2005 - Conventional Chest physiotherapy compared with Other Airway Clearance Techniques for Cystic Fibrosis). you'll find evidence in the form of systematic reviews, but overall, I remember the findings are something like 'chest physio is better than no chest physio, although there is no particular technique which is most beneficial'
- there isn't however, much evidence based upon the effectiveness of techniques in the treatment of adults as you can imagine why.