Did you know the Cochrane database ?

some examples linked below : speech therapy for Parkinson, Stroke or dysarthria in childrens

http://onlinelibrary.wiley.com/doi/10.1002/14651858.CD000425.pub4/full

http://onlinelibrary.wiley.com/doi/10.1002/14651858.CD002814.pub2/full

http://onlinelibrary.wiley.com/doi/10.1002/14651858.CD006937.pub3/full

There is a great source of evidence based information dedicated to speech therapy. It is www.speechbite.com - an up to date database that collects and characterizes scientific papers about treatments according to their evidence level.

Have a look at the ASHA "Evidence Maps" http://www.asha.org/Evidence-Maps/

Also from the Communication Trust there is "What Works" http://www.thecommunicationtrust.org.uk/whatworks

Many SLTs in the UK are now using syntheses of evidence like these to inform their choice of approach.

I think there is a lot of literature on the subject. You can find a sample attached.