On-line tutors - could they produce a 2.0 SD swing?

Following on from the last post - could experienced on-line tutors produce a 2.0 SD improvement? This is certainly something interesting to explore.
We are in our teacher recruitment round and I do come across teachers in other countries with the experience and personality to be able to do on-line tutoring.
They would have to support the subject teacher, know the approaches used by her, and, to use a variant of the Google phrase, do no harm. By that I mean that explanations and approaches should be ones that the subject teacher approves of.
How would it work? Skype is the obvious choice.
How would we keep records so as to pay the on-line tutor? Would it be done on just a retainer with a maximum number of hours "on duty"?
How do we get feedback about usage and quality?
Could this be the support that would make a difference for at least some of those struggling with some subject matter or skills?