LETRS: Weasels or Energy Efficient Light Bulbs?

I am looking for research to support the proposition that LETRS: (a) enables teachers to teach better, (b) enables readers to read better, and (c) is better than other forms of professional development. Easy-peasy, simple-pimple. But, alas alack, I have come up with nothing. Hmmm. Hmmm. How can something that's "research-based" have so little legitimate research upon which it is based? Could it possibly be that ... that ... it's not?